How did Marvel get away with not starting "Avengers: Endgame" with the Marvel intro?
It started with a 2 minute scene that plays before the Marvel intro. Like, aren't they required to start with the intro by the MPA?
There's no legal requirement for a movie to have anything.
Most people contract for certain things, but it's all negotiable. Say a big time director like Nolan wanted to have no credits at all; he'd have to make a lot of concessions to the Screen Actors Guild and the other unions, but he could do it if he really wanted to.
Why did Marvel not put their intro at the start of a Marvel film? Likely because a director convinced someone this would make it more impacting.
Afaik there is no legal requirement to have the those at the start it likely is a contractual thing.
I'm very uncertain about what various groups requires these days but format violations are usually just fines - a famous example of that being Star Wars which got into a fight with the writer's guild. The actual statutory opening credits are pretty few and most of the more visual ones (like the Marvel one) would only be enforced by a private contact between Marvel and the production company... so if Marvel was okay omitting their usual opening crawl there'd be no legal obstacle.
The internal MPA regulations have gotten very lax over time. My understanding is that there isn't a time limit between the opening scene and the opening credits.
It was technically the last half of the previous movie, so the intro isn't required.