If they used a Yamanaka to view who attacked Team 7 in the Forest of Death then they would've been more prepared for Orochimaru. They were already really concerned over the fact that Sasuke had a Cursed Seal to the point that it technically required a sealing master to actually treat it as opposed to Kakashi's band-aid solution, not to mention the example of Anko is a thing to draw concern. Not to mention the national security implications.
They already knew who attacked Team 7. Orochimaru even shows up WHILE Kakashi is doing his thing to Sasuke. They 100% knew Orochimaru was in the village. There was a rather lack-luster reaction, but they did, in fact, know.
Anyways, my initial concern was that it could end up diagnosed with some kind of split personality, or maybe he'd just be brushed off as having an imaginary friend. I dunno. In any case, I actually kinda agree with you on this now; Minato would definitely trust in Shiro's intentions and good-hearted nature, but maybe not so much believe in the information Shiro's trying to convey.
Either of those wouldn't be too unusual. The Kurama Clan are known for their strongest members having a split personality / demon in them.
So, Black Zetsu has motive to mess with Hamura's children, sure. How does Black Zetsu teleport to the moon then? Also I'm fairly certain that Hamura's children died out due to their low population numbers with zero chance of introducing new people to intermingle with the clan (since they were forbidden to interact with Earth's inhabitants) in order to preserve that Otsutsuki strength without it becoming diluted like the earth-bound Hyuga clan. The best way for Black Zetsu to mess with them is to leave them to rot on the Moon, technically, which he probably did.
It still doesn't give reason to believe that Hamura's children are reincarnating like Asure/Indra. We're given zero reason to believe that anyone is reincarnating like Asura/Indra. Heck, it's a Zetsu-plot in the first place for Asura/Indra to reincarnate.
Also, as Kaguya's will, there's absolutely no way she didn't impart the dangers of her clan to Black Zetsu in case they came around; he'd certainly need the knowledge in order to leverage it against said rival clan members in order to either help revive his mother or in order to help drive them off in some way.
Black Zetsu could either take the portal that is in the Land of Fire (that's how Naruto and crew get up there), or he could probably jsut fly up there. Momma could fly, I would think he could do the same.
The Otsutsuki clan were BIG. A few hundred atleast. They were definitely above the limit for long-term viability. Here's a pic of AFTER the war against the Main Family, lots of survivors even if this was literally everyone. They're numbers DID dwindle over time, but they never said why. I blame Zetsu.
There's no reason to believe they WEREN'T reincarnating. Hell, there's no reason to believe that EVERYONE isn't reincarnating. The Asura/Indra cycle actually isn't part of his plan, Zetsu made it sound like they were more pains in his ass than vital pawns, and while they ended up being vital to the plan the plan, on it's own, doesn't require them. Hell, the moment Madara awoken the Rinegan, Zetsu could have finished the plan with Obito alone. Things didn't
work out that way, but it was a way things could have gone.
Thing is, without Kaguya or Hagoromo and Hamura coming back, there is nothing that Black Zetsu or ANYONE else could have done against the Uncles coming to visit. There is no mounting any defense of anything, it would have been game over. So if he knew about them he would have worked EVEN HARDER bringing back Momma.
I think there's a difference between thinking and accepting there's more than one monk temple, and believing that there's other random people transcending death and reincarnating into people without rhyme or reason. To put this into perspective, it's specifically noted in canon that only Kaguya's sons and Hagoromo's were able to do stuff after death; it's also noted that Hagoromo's sons exist outside of time like Hagoromo and Hamura and that they have to specifically choose who they're going to be reincarnated as by observing people on Earth and their traits to pick a suitable person.
In order to reincarnate, they need to exist outside of death/time a la Hagoromo and Hamura. I doubt that any of Hamura's children managed to do the same after they transferred their main powers to the Tenseigan container in order for Hamura to comply with Hagoromo's "just according to Keikaku" plan of not interfering with Earth for a thousand years.
Besides, even if Hamura's children were reincarnating, they could only reincarnate into their direct descendents, which are all on the Moon. Presumably the Hyuga clan are an offshoot of some child of Hamura's that was left behind, and I don't think that said child would reincarnate without major reason like Asura/Indra had.
Where is it noted that ONLY her sons do that? Not in the episode where Naruto meets Hagoromo, they also don't say anything about selecting their reincarnations. Those are the ones we SEE, but that doesn't mean those are the ones there ARE.
They can reincarnate into off-shoot clans. Naruto reincarnated into the Uzumaki, which is an off-shoot of the Senju.
And yet unlike Worm, there's probably only been like a thousand or so Uchiha who've achieved the Mangekyou Sharingan, which means that their special powers are pretty much guaranteed to be rather unique. Besides, it's far more likely that more Uchiha had gotten some kind of teleportation/spatial transposition ability rather than sheer dimension hopping like Obito got. It's like the difference between (in Worm) capes like Strider and capes like Doormaker; there are plenty that can teleport since the mechanics of it are much easier and more likely to come by than dimensional travel.
The MS is very easy to get. I imagine MOST Uchicha, who are alive during War Time, get it. People are dying left and right after all, everyone is losing friends and brothers.
In Worm there are a lot of capes that teleport, there are ALSO a lot of capes who deal with Dimensions. They are equally common. Don't see why they wouldn't be here.
And yet you're ignoring the fundamental fact that Shinobi don't like to disseminate information/proof to their enemies, much less to random Shinobi in their ranks. How do you get everyone to believe you, if you don't have them perform/bear witness to the technique itself? You can't just say "The afterlife is real, we found out by performing a specific technique. What technique? We're not going to tell you because our enemies might use it against us, and there's probably some pies in our ranks."
We have incentive to believe that stuff like that aren't even uncommonly known due to Shinobi culture, and how it operates.
Ninja can't have 100% information security. They are good, but they aren't perfect, and that's before considering the only way to feasibly get 100% is to NEVER use it. Plus, as we see OFTEN throughout the series, Ninja kinda like to monologue about their techniques. You can say a lot about a technique, like it's name (everyone always says the name) and the basics what of what it does ("This brings people back to life as immortal zombies! You'll never win!") without giving people any information needed to recreate it.
Not bending; very specifically dimensional travel is needed, and only if an Uchiha gets the bright idea to use it while in the afterlife's waiting room. Not to mention that he could only grant Kakashi his powers that way due to the fact that Kakashi's chakra system was already acclimated to Obito's Yin chakra from his Sharingan specifically, not to mention that only Kakashi had knowledge on how to operate said Sharingan. Otherwise, Obito's ghost couldn't interact with the world at all and could only stick around for a very short period before being yanked to the Pure World.
What proof is there that obito couldn't do anything on his own, and could only stick around for a very short period? For all we know, he could have just stayed, indefinitely. All he says is that his gift of the sharingan was temporary.
And yet there's no record of immortal zombies being on Konoha's side, nor is there any existing countermeasures to combat that technique in the other villages as demonstrated during the Fourth War (at least until they repurpose other techniques on the spot). If it was known that Konoha had this technique, then Suna wouldn't have met with Orochimaru in order to plan to betray Konoha in the first place, because they thought they had a fighting chance and didn't think they'd be up against an immortal army of Hokages like you seem to like to think would happen, which it obviously didn't. Konoha wouldn't have struggled so much against Iwa and Kumo in the wars if they had Hashirama and Tobirama doing as you say as well. That's not to mention that there's absolutely no mention of Konoha reviving their fallen to decimate armies in regards to other villages in canon, as I'm pretty sure that'd be enough of a huge deal to inform the audience.
Like I said before, it's very likely that there were major problems with Tobirama's unfinished version that Orochimaru fixed with his own after many years of tinkering with the formula.
There's no record that they were there, there's no record that they weren't there.
They didn't have any countermeasures, but they didn't have any countermeasures against MOST of the absurd things that Konoha regularly pulls out of it's ass. Konoha, from all the canon evidence, should have just stomped ALL the other villages anytime War came up, for dozens of reasons. But they somehow don't. Kishimoto is not that good at world building.
Suna didn't meet with Orochimaru. Their Kage lead them to war, and they followed. Said Kage probably made assurances that they wouldn't be facing the Immortal Hokage Zombies (because he was gonna be using them himself).
Not only are are the Hokage a lot weaker as Immortal Zombies (still very strong, but not their absurd power levels of life), but supposedly the other villages weren't such jokes back in the day. I mean, they did manage to kill Hashirama and Tobirama in their prime somehow. Those would be good enough reasons to explain why the Zomb-kage didn't just stomp the wars.
So does everyone know all the techniques of the Rinnegan or do they not? It's quite obvious to me that they don't, but for some reason you think that the Sage very specifically explained and had reason to use all of those abilities of his Rinnegan in a public setting when he was teaching people Ninshu and not fighting up against the only threat of the time that would need such abilities used (Kaguya).
They know them, they just think they're legend. And no, the Sage probably didn't, but all the other holders of the RInnegan probably did. Again, Madara wasn't the only one, just hte most recent.
That was a forbidden Uzumaki clan technique that I doubt many people were even aware of, considering the cost of using it. It's far more likely that they wiped out the Uzumaki due to their continuous rise to power through their vitality/longevity and their ever-advancing sealwork.
The one that doesn't kill you probably wasn't forbidden, since it covered up the part that makes it forbidden. And, well, summoning Death Itself is kinda eye-catching. If the enemy had ANY survivors from a battle where it was used, word would spread quick that they summoned the fucking Shinigami.
And like always people will demand proof, except in pretty much all cases they can't due to Shinobi culture.
"Dude, look at this footprint on my chest. That's Hashirama's shoe. No one else wears shoes like that!"
Proof would be easy to come by, unless Commanding Officers just refuse to believe there their subordinates.
Hashirama was a Shonen protag in canon, and he was obviously a pacifist. I mena, his entire (semi-succeeded) goal was to stop the war between clans and set up a village where children don't have to fight.
Once more, I refuse to believe that it wouldn't be noted somewhere in canon, especially during the Fourth War by some reminiscing Shinobi or Kage, about immortal Hokage being sent to the front lines in previous wars. That's absolutely absurd that you would think that to be the case.
Itachi was also a Pacifist. Just look at what he did to
pretty much everyone he was related to. In Naruto, Pacifism doesn't mean "I don't kill people", it means "I kill people only when I need to", which as a Ninja, comes up fairly often.
A LOT of shit isn't noted anywhere in canon. Like, pretty much ANY of the details about the previous wars. Or what the Uzumaki were up to, it was some serious shit, but beyond the stuff surrounding the personification of Death, we know nothing.