If Tony and Nebula are barely on the Benatar before Carol shows up to rescue them, what's the deal with that shot of Steve (in the black undershirt) and Nebula with tools repairing a part of the ship supposed to be from?

Marketing deception.

How does Scott's van go from the top of a parking garage to "dusting in some storage facility"? Did anyone know he was there besides Cassie? And if not, did the individual(s) who found the car not bother to try testing the cables to see if they could bring him back, if it was as easy as pressing a single button? That literally makes the speculation of Scott doing something badass to get out of the Quantum Realm (harnessing the energy in an unintended way/finding a portal out) stupid because he needs a deus ex machina in the form of a rat to get him out.

The person who found the vehicle wouldn't know what device they were looking at. You're right though. Some of these things don't make sense but I believe they fall in the suspense of disbelief territory. Yep, a rat was indirectly responsible for saving the day.

So, Nat kills herself out of love for... herself? (uh) Clint? (super weird?) And they just kill her off less than halfway through the film with no fanfare? They want to make a prequel for her? What? Didn't they see Solo?

I don't think the soul stone sacrifice works as straighforward as everyone believes. Yeah its weird they would make a Black Widow movie posthumously for the character.

So they've not only undone any character development Loki had with Thor et al. in the interim between the first Avengers and Infinity War, but they have him walk off scot-free with the Tesseract? No one stops him? That just sounds stupid.

Again more confusing stuff. His fate is ambiguous I guess? Is he dead? I don't know. This time travel shit is fucking confusing.

Wait, so they kill off actual post-Snap Thanos in the opening, who's either given up or is weakened by the gauntlet's power, and the final fight is against a Thanos from five years earlier, at a point when he has barely kknows who the Avengers are? And he finds this out because Nebula fucks up and shows him video of the events on Titan? How is that satisfying to the audience if this Thanos barely has any idea of what's going on in the first place?

He doesn't barely have an idea. This Thanos still believed in wiping out half of the universe's population and was still seeking the stones. Nebula's hologram or whatever would have made him aware that the a group of superheroes were using time travel to undo his plan thus making Past Thanos aware that he had succeeded in the future.

If Thanos takes out the Avengers facility in one blow/explosion, why is the final battle (as per the leaked footage) set in an area with different piles of debris scattered around as far as the eye can see? Is that all from the Facility?

Thanos and his forces probably wrecked the whole area. He was pissed.

You state, "Rhodey and Rocket are nearly killed by underground floods but Scott saves them. Clint finds the gauntlet underground and carries it around (not putting it on), always on the run from Thanos' army. Thor, Tony and Steve fight Thanos. Thor has his armor back on, his beard braided and looks much better." Yet the leak shows a scene with Hulk trying and struggling to stay above water in a confined space while Tony/Steve/Thor fight Thanos. What's that supposed to be?

OP said in another comment that Hulk was struggling under rubble. She also mentioned something about underground flooding hence the water scene with Hulk in the leaked footage and the water scene with Hawkeye in the trailers.

So, new-Nebula (pretending to be old-Nebula) fucks with time by teaming up with old Gamora to kill old-Nebula? How does that NOT cause a giant plot hole? They've made it so that Gamora will never meet the Guardians!

Like I said time travel shenanigans are perplexing. They are many other scenes described that would fuck up the timeline. Suspension of disbelief I guess.

Hulk doesn't get his rematch with Thanos? Really? After the setup at the beginning of IW?

I believe OP said that Professor Hulk due to him having Banner's qualities would be less unwilling to engage in combat. I'm sure he got his rematch though or at least contributed in the final battle.

If Steve voluntarily goes back into the past to live out life with Peggy, why does he not rescue or do literally ANYTHING to prevent the events of Civil War (rescuing Tony's parents/dismantling HYDRA before they corrupt SHIELD) in the intervening 70-plus years? And how would he stay incognito if he's dating/married to the co-founder of SHIELD?

Because doing anything of such would ruin the timeline so he couldn't to ensure victory according to time travel explanations. Things had to happen that way.

So wait, Peter is now five years younger than the rest of his classmates? And those classmates lived through the entirety of those five years - AND are still in high school? And they want to go on a field trip? WHAT????

The age thing is confusing. Might be simplified in Far From Home. I think OP or another viewer said that only the superheroes remember what "happened".

If the snap is undone, what happens to the tens of thousands/hundreds of thousands of people using transportation/already in dangerous situations when the snap occurred, and died because of their transport crashing/other extenuating circumstances?

The Snap restored things to how they were before well ... before the Snap.

No Carol/Fury reunion? After everything they went through, it doesn't seem possible that this just gets relegated to an off-screen thing.

Good point. Apparently not since Nick only appeared for the funeral. Maybe they had an extremely brief interaction there?