The rules of time travel are pretty damn easy to follow in this movie, IMO. Since Doctor Strange, they have set up the concept of multiverses (The Strange movie literally says "multiverse" MULTIPLE TIMES over the course of it). You enter a time from the past, and it is impossible to change the future in your own timeline/multiverse. Once you try to change the events in the past in attempt to change the future, you're essentially creating a new multiverse (separate from the timeline you came from). It is seriously not hard to understand it in the movie, as the movie takes its time explaining it twice. lol.
And no, Marvel wasn't shitting on movies like Back to the Future. I mean, Marvel basically used Back to the Future II as something of an inspiration (maybe even a ripoff to some people, lol) for the second act of the film, after all. They were just mentioning all other famous time travel movies to say "guys, we're doing something a little different and making our own rules, so disregard some of the things you've seen about time travel in other movies when you watch this."
And their complaining about how "none of the heroes used any of the individual Infinity Stones." Um. All of them went into the Quantum Realm at the same time? And all of them came back at the same time? And their one goal was to simply "un-snap" the lives that vanished? Plus, the first Guardians of the Galaxy very well established that not many lifeforms are powerful enough to even hold an Infinity Stone (which is why, you know, Tony dies using the Gauntlet and why Star-Lord needed help with the Power Stone even though he was a god at the time)? Guys, are you even paying attention to the film or the films that came before it, setting all of this up? lol. Even if someone like Thor wanted to use one of the Stones, he literally had no time or chance to, as Thanos attacked the Avengers base seconds after Hulk used the Gauntlet (and the Gauntlet got buried under rubble as a result of the surprise attack).
And if the Avengers just went into the Quantum Realm to get only the Time Stone, do any of them actually have the power, knowledge, or strength to use it? Doctor Strange had to study a LOT in his own movie to understand how the damn thing worked. And I'm sure Thanos did the same as well, since he's known about the Stones for a very long time. Plus, it seems that only one person can use the Time Stone at a time, meaning that if time travel worked with the stone, then only one Avenger would be able to time travel ... Which means that only one Avenger could go up against Thanos ... Which means that the rest of the team would need the Quantum Realm anyway to join that one Avenger using the Stone to help them. lol. It makes no sense to "just use the Time Stone," for several reasons.