Regarding all the time travel questions, I need to watch it again to confirm, but I honestly understand it that the pasts they travel too are just all separate realities. The comics have a multiverse and so too do the movies. There can basically be an infinite amount of realities set in 2012 and 2014 etc. This is why "the past" doesn't affect the present.
When the Ancient One was talking about branching off if an Infinity Stone goes missing, I think she meant from her realities intended flow. Each reality has 1 time flow and that can't be changed, remove an Infinity Stone and that reality crumbles. In terms of time, Steve probably returned each Infinity Stone 5-10 minutes after they were taken.
The people asking why they don't go back in time to resurrect Loki/Heimdall/Gamora/Vision/Natasha, are these the same people complaining about "no stakes" in the MCU films? I hope it's not because if they are, they are complaining just for complaining sake. I'm fine if in 5-10 years they find a way to bring them back but not in the next/same film that they die in. It would come across as incredibly cheap if they just easily resurrected these characters. The fact they are staying dead is a real impact and it was high stakes.
My explanation above is a good enough reason for them not being brought back if you ask me as well.
*EDIT*
I'm incredibly impressed that a time displaced character is in the films. I honestly thought something like this would never happen.
It's like the 60's Jean Grey in the more recent comics of X-Men.
*EDIT 2*
Does this make more sense for you guys?
https://marvel.fandom.com/wiki/Glossary:Time_Travel
"A time traveler does not truly travel straight backwards or forwards in time, but backwards or forwards and a bit off to the side to a divergent timeline now running parallel to one's timeline of origin. Since this timeline will have been identical to the "virgin" timeline until the moment of divergence, there will be virtually no differences between the two timelines until most time travelers have no reason to be aware that they are not on the "virgin" timeline.
If one travels a second time to an era one has already been to, one will not materialize on the "virgin" timeline nor the timeline diverged by one's previous trip, but a third timeline diverged from one or the other. A time traveler can never travel back to the exact same timeline more than once. Again, since the second and third divergent timelines are identical until the time traveler's arrival, they will be indistinguishable at first."