If Oro doesn't care before, he won't care after. This won't make him care one way or the other
You're assuming Tsunade would care if we choose to commit suicide by Oro, most likely she'll tell Hazou he's being unbelievably stupid and refuse to play along. Possibly throw him in volcano for being possessed.
The "pre-existing conditions happened" scenario is exactly what I had in mind. Just enough plausible deniability that no one is going to go toe-to-toe with Oro over it. Asuma would be pissed probably, but he'll rationalize it away as being Hazou's fault for putting himself in that position in the first place, which it 100% would be.
Frankly, we cannot minimize the probability of sabotage, the only person who could possibly contradict Oro about this is Tsunade and I don't think she cares that much about Hazou to bother. Do you really think Tsunade would kill (or attempt to kill) Oro over Hazou's life? I don't, she's got better things to do.
Edit: I wanted to expand on the last part a little bit. Imagine that Hazou establishes all of the suggested safeguards, Tsunade, Clan Heads, ninja at large. Hazou dies anyway, Oro says "pre-existing conditions happened". Now our allies (in the best case Tsunade, ISC, Asuma, Goketsu, and KEI) are faced with the choice to attack Oro or continue on as before. If they attack, many of them will die and Oro will almost certainly get away, if they don't he'll continue to serve as a deterrent for Leaf. The best option is for them to accept the cover story, no matter their private feelings. Oro knows all of this and he can act on it in advance.
This might work, but I think it has a solid 50+% chance of ending in death, that's way way too high for my taste
I think your model of the safeguards and Orochimaru's response to them is incomplete. You say that the safeguards won't work because nobody would actually pull the trigger on trying to kill Orochimaru over Hazou, but even supposing that's true I think it misses some very key points.
Orochimaru has a finite amount of social capital in Leaf. Sure, he's a key element in keeping other villages from attacking Leaf in its moment of weakness, but too much trouble and it'll be harder to keep Leaf safe with him around than without. There is an economy at play here, and Orochimaru knows it. It's why he's very deliberately playing by the rules and accepting cooperative bargains that reduce the need for him to do things costly in reputation. He's no fool, and he knows that his time in Leaf will go much more smoothly if he doesn't make people more nervous about him than they already are.
Hazou is a Clan Head who has very recently demonstrated enough support across Leaf to make the Hokage back down on executing him for obvious treason. Even if accidentallying Hazou isn't enough on its own to get Asuma to pull the trigger, it's an
extremely steep cost on a currency he's clearly been paying attention to. If he's willing to accidentally one Clan Head in metaphorical broad daylight, even if it's just Hazou, every other Clan Head will feel significantly less comfortable with him in their village.
This isn't consistent with the playbook Orochimaru's been operating from since his return. Again, we are not the center of his universe, we're just one more bloodline to look at, who happens to be attached to a person it'd be
very inconvenient to accidentally. I could also mention the outside chance our pseudo-familial connection matters some to him, but I don't think I actually need to.
(Also, on the flipside, imagine how much
more comfortable it would make people if someone like Hazou willingly entrusted themselves to Orochimaru and he
honored that trust? That's valuable social currency in and of itself, in a village that still isn't too comfortable with his presence)
And if somehow none of that sways him, if Tsunade's there then her presence as a
potential S-Rank fight is deterrent in its own right. Sure, maybe she wouldn't actually pull the trigger if Oro went for the kill, but maybe she would, and is that a risk worth taking? We're just a kid with a bloodline, after all, who's already giving 90% of what he could ever want out of us of our own free will.
When I think about Orochimaru's perspective I see two options: the first is taking the opportunity presented to him on a silver platter, showing off how reasonable and not-monstrous he is to the rest of Leaf while getting a good research opportunity in the process for no cost. The second option is to risk an S-Rank fight in his home and throw a ton of social currency down the drain, making future cooperation harder and bringing him significantly closer to the point where Asuma decides it's just not worth it anymore, all for the sake of the 10% of value not given to him for free. Given how he's been behaving since he rejoined Leaf, I legitimately can't see him taking the second option over the first.
Alternate scenario: he learns plenty in his nondestructive investigations of the Iron Nerve, but there's far more that he could do with destructive tests. He's more interested in us than ever.
I think that the strength of the 'ignore him' strategy is, at least partially, that its outcomes are as low-variance as possible given someone who we cannot model well. They aren't necessarily great outcomes - we can't attempt to leverage his tremendous and useful skillset for fear of attracting his attention - but thus far, it seems like he's happy to leave us alone as long as we don't bother him, and any deviation from this pattern increases the spread of outcomes dramatically IMO.
You're right that this does escalate the timeframe, brings us to the conclusion of the Orochimaru fear arc sooner one way or the other, but I would advise against getting complacent with the assumption that since he hasn't done anything
yet that we're safe from him. An idle worry I sometimes toss around is that we're just somewhere on his to-do list, and once he strikes off everything above us he'll find some way to get us that doesn't even cost him any rep. Maybe we pick a chakra beast suppression mission one day for some casual Uplift and just never come back because Oro snuck out and abducted us and nobody will ever know. Going to him makes it a battle on our terms, where we get to deny him any opportunity to avoid broadcasting his choice to all of Leaf.
And I recognize that there's a
chance that the potential that experimentation on the Iron Nerve will reveal potential death experiments that raises his interest rather than lowers them, but personally I find that a rather unlikely chance. And not to mention, once we have public vivisection dealings with Orochimaru, it becomes that much easier to suspect him if we disappear mysteriously, meaning that even in the unlikely case that he becomes more interested in us despite already exhausting most research potential I don't think the actual probabilistic risk would change much. Thus, overall the probability distribution spans more or less from 'unambiguous win' to 'uneasy neutrality', which still makes it a win in my book even if we don't get anything in return.
Overall I do hear your concerns but I just don't think they hold up when shined against what we
actually know about Orochimaru and Konoha. My position is not that there's a zero percent chance this goes wrong, but rather that it's safer than you might expect, and maybe even safer than what we're currently doing. I know I'm working an uphill battle here since the default position is to avoid the scary essie, but I hope you can see that I
genuinely believe this is not as dangerous as initial instincts might suggest. Between the list of deterrences we can establish and Orochimaru's incentives and known behaviour I think it's actually highly likely that we survive.
