I think
redoubt27 and
TonyMurray captured most of what I am about to explain. First of all most of the corruption will be focused on Sarah because at least in my mind Alex will get to that state of mind much quicker compared to Sarah. For example if Sarah tells Alex "hey let's ask Jessica/Eva/Emma/Rachel for a threesome" I think Alex will need much less convincing to go through. But if Alex says "let's ask Neil/Erik/Dwayne/Vincent for a threesome" Sarah will most likely turn him down unless she is exposed to these ideas first. Or at least that's how I picture the scenario.
And about Alex becoming a doormat, this only happens if you let it happen. Every interaction when Alex expresses his opinion on something Sarah has the option to follow it or ignore it. (This will be true with Alex events too like when Sarah says she doesn't like the idea of Alex having lunch with Emma, but he can still do it). So if you are playing route 1 and route 2, Sarah is supposed to respect Alex's wishes and respond accordingly (or vice versa). You have to work as a team. If Sarah (or Alex) are going to go against their partners wishes then they will automatically become a silent observer (or doormat). What is not going to happen is their partner pushing back leading to disagreement and fighting. Why not ? because this game is not trying to emulate the human condition, but rather allow you to follow one of the four routes based on your liking. So if an interaction is going to take a lot of development resources without advancing the game on one of these paths, they will be dropped. There can be disagreement or fighting but near the end of the game.
1) Alex have already mentioned his opinion. If you are playing route 1 and 2 Sarah should be on the same page. If Sarah goes against Alex's wishes you are making the choice to make Alex the silent observer. What Alex is not going to do is push back and argue (or go and fight Neil) because that will take a lot of development resources and make the game slow without advancing on the routes much.
2) Same thing. If you want to avoid Neil you will forget the video and move on with your life. Because going to the HR or cops would create too many branches in the story that will unfortunately cripple production.
3) I have explained why this happens before and how this doesnt affect Route 1 and Route 2
4) I think this is mostly answered.