Thing about House Elves: Dobby is able to go to Malfoy Manor in Book 7 because he's on the whitelist. Assume Malfoy senior just never took him off. Probably he didn't think it would be a problem due to Dobby never wanting to return, or he assumed Dobby would just die like elves separated from their families are known to do. As for Kreacher hunting down Mundungus, Dung is unlikely to be anywhere under strong wards, and Kreacher is a
Black Elf, which means he's probably empowered by some nasty fucking rituals meant to allow him to better serve the house of Black. But even that isn't enough for him to invade places that he doesn't have a claim to being invited to, so long as the wards can actually keep him from coming in anyways.
Privet Drive Blood Wards doesn't stop Dobby because Dobby truly, genuinely does not mean Harry harm, and is in fact (in his own twisted way) trying to help him. He also doesn't have orders to be there from anyone bearing ill will against Harry. If you subscribe to the 'Dobby stealthily links himself to Harry' piece, then you can have him show up on Harry's command if called inside Grimmauld. Elves canonically seem to hear their names when called from a place they can go to. And we know Hogwarts has some Apparation whitelist for the Hogwarts employed Elves.
Elves just don't have the super mobility and powers fanon states they have, because there's only one real instance of an elf being actually that impressive, and that was Kreacher hunting Mundungus, and again, Kreacher is a little monster, a true titan of his kind. Mundungus is taken by surprise and isn't that great of a wizard to begin with.
It also helps if you say that House Elves have a poor staying power, and run out of stamina quickly, so they can't take on lengthy magic intensive tasks without needing to slow down and rest.
What else? The Basilisk? Pick whatever, the movies should follow the books, and the books have nothing to state about the status of the corpse post book 2. Harry never goes back down there again (which is a shame, considering all the stuff in book 4, but he would have needed to bring a broom assuming the fanon / canon 'ask for stairs' thing (Ginnymort was getting out of there without a broom SOMEHOW)) to see for himself.
Anti-tech aura? We get told that magic interferes with technology, but we never actually see this actually happen. My own headcanon is that it requires an extremely magical location of ludicrously high density to get that aura, and that there's typically only a few sites like that in any given magical zone. Hogwarts, Saint Mungo's, Diagon Alley, the Ministry of Magic. The main school, the main shopping area, and the locations of the myriad Ministries in any given magical ICW area. Hogsmead doesn't have the density of magic required to create this effect, and obviously neither does Godric's Hallow. And even then, this effect can also be contained by a very simple magic: Expansion Charms. So you can create a magical super density area in the middle of London and it simultaneously isn't detectable by technological fizzle because it's already accounted for. If there's one thing Wizards are genuinely good at, it's keeping their secrecy.
Owl bombs: By all indications that absolutely would work. There pretty much has to be owl redirection magic though, otherwise is creates plotholes like 'how does Sirius hide if you can just send an owl after him?' As such getting mail sent to purebloods is first collected in a safe mail collection room which is warded against explosions and has alarms for hexes, curses, and poisons. And then House Elves check it before bringing it to their masters. Harry has lived his entire life under very strict owl redirection wards controlled and placed by Dumbledore, but the magic has enough adaptability that Harry can receive mail from those he doesn't suspect as being against him. And you can do the whole 'Goblins have been sending you yearly reports you've never been getting' that way too. You can even excuse Albus by having him go 'oh, shit, right, I needed to adjust that now that you're not ignorant of magic anymore'.
Magic Immunity / Resistance: We see several creatures that logically are only dangerous if they carry some form of magic resistance (anything 4 X should have some resistance, anything 5 X pretty much HAS to have resistances), but we also never hear any true magic immunity among creatures. Even Dementors aren't totally immune to magic, though they seem ludicrously resistant, along their abominable false-life state. So long as you're not totally magic immune (which nothing we know of is) you'll still die to an Avada. Dementors and perhaps their 'cousins' Lethifolds are not truly alive, and so cannot be killed or even destroyed by any known method in the Wizarding World. (I think Fiendfyre should be capable of it, actually, so long as you could get it past their icey aura without losing your will and Fiendfyre rebounding on you for it.)
Muggle weapons: In the same manner that Wizarding medicine and healing can fix essentially anything that isn't outright dark magic, their most basic defenses that even third years are supposed to be able to cast would stop anything short of a rocket launcher missile. And considering how tough Wizards are to survive bludgers based off of bludger calcs, they're probably shockingly resilient to small arms and even medium arms fire anyways.
Sinister Timing: By accident or on purpose, there's a
lot that happens to get Harry landed with the Dursleys. First, Voledmort and Wormtail show up. Voldemort presumably forces Wormtail to come with him so that if Wormtail is lying to him he doesn't get an opportunity to run, because finding a common brown rat would be troublesome even for the Dark Lord. Voldemort attacks the Potters and gets disintegrated on the rebounding Avada, and blows a hole in the roof in the process, but his robes and wand are left intact. Wormtail investigates, snatches the wand, and flees. Seemingly within 10 minutes Snape shows up to cradle Lily's body, cry, and feel bad for himself. Snape then leaves as Sirius enters the scene, audible from a distance due to his motorbike. Sirius retrieves Harry and either runs into Hagrid literally as he comes out the front door or he sits on the porch with Harry and has a mental breakdown. Hagrid manages to get Harry away from Sirius due to being insistently sent by Dumbledore, who has strong reason to suspect something's up with Sirius. Especially if you go by the 'Lily doesn't trust Dumebldore' fanon idea where she gets Dumbledore to cast Fidelus for her them using Sirius as secret keeper, and then as soon as Dumbledore leaves she takes down the Fidelius and having just seen it cast makes a new one using Peter without telling Dumbledore about the swap, which would leave Dumbledore understandably convinced the Fidelius was centered on Sirius. The timing window here is
tight, but Dumbledore already has everything figured out for putting Harry with the Dursleys. It does end up looking sinister, though I choose to believe Dumebldore had simply already figured out an itemized list of what he'd do for Harry / Neville should X Y and or Z occur, so that he was prepared for any scenario. Being properly paranoid isn't sinister, just kind of creepy.
Pouch: It's not moleskin. It's
mokeskin.
Dumbledore not being sinister: There's a very convincing fanon theory floating around that the entire thing with first year was Dumbledore trying to trap Voldemort's shade in the Mirror of Erised, and that Harry actually screwed up this plan by going to try and keep Voldemort from getting the stone. Obviously Dumbledore never brings this up because he doesn't want to tell the dumb 11 year old well meaning child he screwed up everything by trying to help.
As for Quirrel being such a shit wizard? He's not, in fact he's pretty good, which is how he's able to do nonverbal magic so easily for stuff like throwing up walls of fire and Incarcerous.
Voldemort isn't aware at that point in time exactly what protections are on Harry, so he has Quirrel cast no spells directly on Harry he'd care if they were supercharged and reflected like his Avada was. And then choke the kid out, because if it's not magic, it's probably a loophole in the protection. Until it ends up being a 'whoops, that didn't work' issue.