For people who don't like Alternate timelines here you go with a batshit insane theory that I thought about. Since Marvel Comics like to use pocket universes I will use them here.
I personally prefer the alternate timelines theory. Because there's way too much Quantum physics involved and too many hints at a multiverse for it not to be.
This is only a thought experiment because it's fun.
If we look at the Fine-Tuned Universe "The proposition that the conditions that allow
life in the
Universe can occur only when certain universal
dimensionless physical constants lie within a very narrow range of values, so that if any of several fundamental constants were only slightly different, the Universe would be unlikely to be conducive to the establishment and development of
matter, astronomical structures, elemental diversity, or life as it is understood."
Applying this to the Marvel universe and how taking any stone can destabalize the Single Timeline and only putting it back would correct the timeline. We have to take the Fine-Tuned Universe theory and have the stones "6 singularities" be these "certain universal dimensionless physical constants within a narrow range of values".
Now changing any of these values by the slightest amount can cause a completely different universe to exist where life doesn't exist at all, gravity is completely different, etc.
My guess is the Mind Stone stores these default values of the Marvel Universe. While the other stones maintain these specific default values to their specific area. Time maintains the flow of time values, space maintains the expansion of space values, soul maintains the soul world values, reality maintains the basic physics of the universe, Power Stone I don't know, maybe maintains the link between all stones) allows you to change these fine tuned values is the Reality Stone. However as witnessed in the movies, these stones have a limited range if not used in conjunction with all the stones.
In Thor 2 Dark World these "Fine-Tuned" values can be changed so the the wormholes that connect all 9 realms (planets that I guess are the far corners of SpaceTime), can be used to interlock the wormholes permanently allowing the changed values to spread across the universe to create permanent darkness. Male moth has to wait for the Wormholes to perfectly line-up as the Reality Stone by itself has a limited range, so the portals lining up would create a shortcut to the entire universe.
Now applying the Fine-Tuned universe to the 6 stones means each of the 6 stones have these default values from inception to allow the creation of the Marvel Universe as the way it is up to the point they are used to change the default values or the end of SpaceTime. So as long as the 6 stones remain in the universe they can maintain their default values all throughout time and space in that specific universe until they are used or removed.
Now let's look at how Dr. Strange used the Time Stone to rewind a pocket of Hong Kong let's say like 10 mins. Now that pocket of Hong Kong is 10 minutes behind the natural timeline of the universe but because the past is the present and the present is the past, that pocket of Hong Kong still flows in the present of the timeline. This Pocket Universe is the size of the few blocks of Hong Kong and those people will have experienced a 10 minute time skip.
The Time Stone was used to change the default values, however since all 6 stones still exist at that specific moment SpaceTime in the present, This pocket universe the size of a few blocks, now merged with the main SpaceTime to maintain the default flow of time.
So when the Avengers travel back in time and remove the stone from that point in time it is happening in the present, not in the past. However for this to occur it would have to happen in another point in Space in SpaceTime, not just time.
Now remember when Scott says the 5 years was 5 hours. This is the Dr strange syndrome, those people in Hong Kong experiencing a 10 min time skip, Scott experiencing a 5 year time skip. These time skips only happening in a small radius.
Hong Kong it is a few blocks, Scott is the size of the plank scale. (SpaceTime).
So for the Avengers to travel back in time the reason they have to go through the time portal is because the time portal has to shrink that point in timespace to smaller than the plank scale creating a pocket universe in the Quantum Realm. As long as the stones exist in that pocket universe where they are supposed to be that pocket universe will always only remain a pocket universe because all 6 stones are there to maintain default values.
So when Hawkeye first travels back in time for 5 seconds, in the main SpaceTime at that point in time there is a pocket universe that existed for 5 seconds because no stone was removed from that point in SpaceTime. That pocket universe is only as old as Hawkeye existed in it which was I think a few minutes.
However because none of the stones were removed from that specific point in spaceTime, the stones can erase that pocket universe from existing and creating a new multiverse universe.
Now when the Avengers travel back in time they are also creating these pocket universes. However they are removing the stones from them. So these pocket universes have the potential to become new multiverse universes. Because not all 6 stones remain at that specific point in SpaceTime to maintain the default values of the main Universe.
So really Cap does not need to return Mjolnir and he takes it with him cuz it's his now to use in case he runs into trouble returning the stones to those pocket universe in SpaceTime.
So 1970 pocket universe is a few minute old pocket universe. Because the moment Tony and his father leave the storage are would be the point Cap returns the stone. However that pocket universe would exist in the Quantum realm, for however many days in the main SpaceTime it took to defeat Thanos, have the funeral, build the Time machine, and return to it. So returning the stone would erase that pocket universe completely once Cap leaves as the stones now exist all throughout SpaceTime to return to default values. Removing the need for a new multiverse universe to be created from this pocket universe.
So when Cap travels to the 1940's to spend his life with Peggy, he exists in a pocket universe for that instance which is the smallest measured time known to us is the atomic second. However that pocket universe is, let's say he froze in 1945, it is 2023. So that pocket universe which is 78 years of SpaceTime only existed for an atomic second in the main SpaceTime.
Now because that 1945 pocket universe was allowed to exist until the present of the Main SpaceTime, it would merge out of existence as long as during those 78 years in the pocket universe, none of the stones were removed from any point in SpaceTime, that contradicts the main SpaceTime continuum, Once the pocket universe reached the present, the stones would revert to their default values merging the pocket universe out of existence with the main SpaceTime. Hence bringing Old Man steve to the Main SpaceTime since that pocket Universe would have a time skip of an atomic second.
So in the end the Time machine is designed to create pocket universes and the gps is synced with it to travel between these pocket universes.
TLDR better to read whole thing too much nuance to summarize. But just think pocket universe that exist only for the time spent in them as old as the SpaceTime allows them to exist, before being erased as long as stones exist in SpaceTime continuum at that specific point that doesn't contradict their positions therefore not needing to expand that pocket universe out of the quantum realm into a multiverse universe/branched timeline. Therefore reverting the pocket universe's default values to that of the stones in the present of main spaceTime to correct it to default values.
Anyways so when AntMan shrinks to subatomic size he is creating more dense matter than a giant star collapsing onto itself to the point of creating a black hole, He is shrinking trillions of atoms to a size smaller than one atom and apparently shrinking forever. So how is he not creating a black hole? I wonder if pym particles can reverse a black hole.
