Interesting questions Alien; I think both, in their way, boil down to Time not necessarily being linear because the time travel in Endgame is achieved via the Quantum - as opposed to any other way to achieve time travel.
I don't think we need to worry about spoiler tags, they're on the thread title. Fair warning I think.
The Quantum effect for me means a roll of the dice. If you remember Antman and the Wasp, they had to track Janet Van Dyne down via her message to Scott before the 'probability fields shifted'. Going by your idea they would just have had to look thirty hours from when she got stuck and they would have found her. I tend to think instead that both the time spend in realm (entry vector) AND the exit vector are made random by the Quantum Realm and that would have been the case for Scott, except that he was sent in on purpose, perhaps with an upgraded suit that allowed the X-Con time van to track him and get him back, perhaps by isolating one or both of those vectors. (Even if both vectors aren't random, I think we'd need a third example to know if the relationship between entry and exit vectors is a linear one or if one vector is a square, cube or other power of the other). In the end a rat chose the exit vector but that doesn't necessarily affect the entry vector.
The Thanos question is more a question of dimensions I think. We're used to picturing time as being totally linear, one past affecting one future, the Back to the Future model. But in the sliding door situation, every time you make a decision both choices play out in two different dimensions that are created when the decision is made. I think Endgame uses the sliding doors scenario because the statement is made that you can't change your own past. The Avengers that returned to 2012 had already experienced Thanos' snap in the dimension they came from, that was done and can't be undone. What they ended up doing was shifting into a dimension where they could take the stones and not affect that. So let's say in 50 dimensions out of 100 the time stone was never in the Ancient One's hands. All you need to do is pick one of those dimensions where she had it - but other to your own (1/100 leaving 49 targets) - to leave your own original dimension unaffected?