Spoilers ahead
1. I don't understand the first question? I suppose you'r talking about who killed Sydney - (I think) Chase killed Sydney while being possesed by the mirror man (TJ route) or at least partly possesed/ influenced.
2. (It is kinda obvious, it was mentioned many times, I will try to expand it) The histeria is changing people, it affects differently on everyone: TJ- being chased by monster, Leo- gets abusive etc. The point is that histeria affects onto your biggest fears and problems bc it "feeds" on someone else's pain and especially burdened secrets. The only way to last histeria is to survive or reveal to everyone your secret that you have been hiding.
3. There are several monsters that are actually mysterious and we don't get answers on that one. However the "main" monster that attacked Chase group (cars accidents in Flynn and Leo routes), saved Chase from Brian, have been seen by Jenna through her life when she was sad or angry was Flynn. After Flynn route the gila was burnt and become the monster that travel through time (that's why the monster appear in many routes and to many people. Probably he is the reason why Echo doesn't have "true ending" everything is and isn't happening)
4. Again idk if it was a question? Lest say it was and I will make it quick- histeria materialized Leo's tulpa (tulpa is imaginary friend that is independent of you, has it's own emotions, desires etc although it is still in your head)
PS: If I'm wrong pls correct me
Hope I helped. If you don't understand smth just ask, If you don't get something it doesn't mean that it's stupid. I adore this game just because reader has to make effort to understand things (I don't say you'r stupid, at first I was confused as well, but I have read all VN, explanations and spent a lot of time to make all gears inside my head about what the fak I have experienced, you simply didn't :P )