The butterfly effects would add up and and any zygote formed would not be the hitler-as-we-know anymore, since it would be a different combination of sperm and eggs.
Who needs guns when you got a time machine? Don’t like your highschool bully, just bump into their parents back in time. Or you know, “bump” ( ͡° ͜ʖ ͡°) into their parents.
Not at all. There is many ways to rationalize time, nothing is settled at all. The “settled theory” you talk about would create paradoxes, if time travel is ever made real. And paradoxes don’t work well with reality.
There is actually a fairly common way to rationalize time that is the opposite of what you’re describing: Time is entirely a construct, there is no past, no future, only the present. Take away all of humanity’s memories and the past doesn’t exist at all.
There’s also an understanding of time that says it only goes forward, making time travelling to the past impossible.
If you don’t like something that happened to you today and go back to 2000 to change something that change that you made in 2000 had already occurred in 2000 when the thing that you didn’t like happened in 2025. What happened in 2025 happened after the change you made in 2000.