Since open appears to be the place for movie threads, and "Far From Home" (doesn't) deal with the multiverse created by the snap...
I think Professor Hulk messes up when he says they can return to the "instant before" the stones are taken to fix the timelines that are broken by taking the stone.
Going into the past of a timeline does not alter that timeline, and instead creates a new timeline. Anything done in the split does not alter the original.. until you return to the "tip" of the timeline and bring stuff from the other timeline, like the stones.
So, if they were to return to the instant before the stones were taken, they just "split" the new timeline again, creating a 3rd timeline.
Original timeline avengers return to New York during Avengers 1 time, creating a 2nd timeline. They take the time stone and Loki takes the space stone. That timeline gets trashed because the time stone is not there to prevent the conquest of the villein of Dr Strange from destroying our universe.
Professor Hulk says they can return the stone to the instant BEFORE it was taken, but WRONG! That splits to a 3rd timeline, because before is... you know, the past of that timeline, which we JUST established creates new timelines instead of altering the one is split from.
What professor hulk should have said is the instant AFTER the stone is taken. Jumping to a timeline's tip doesn't need to split it.
Hulk takes the stone from the Ancient One, pushes the return button, and poof... poof... Capt America shows up an instant AFTER it was taken and hands it back to her. No 3rd timeline because no one went into the past of the 2nd timeline...
Which still may not work, because it could be the act of moving between timelines that creates the split. IN which case, there would be a timeline where the avengers did not undo the snap, and like 3 others they created going back, then the one they did fix the timeline, and then like 3 more, more when Cap tries to put the stones back and instead just spits more splits....
