Could you pls write things you don't understand? We might help you find answers. This game is amazing from that point of view, it hardly ever say smth flat out but we can get answer from different routes
Well, i cant understand who is... who drowned as they could say, im sorry i cant remember his name.
And... i dont understand why whole town at the and of week, bein insane
Also, i dont understand who's monster.
To be honest, i got creeped out, when i saw Chase doppelganger at Jenna route 
that everyone see, i was like... 'what the fuck?... how the fuck you appear in front of everyone in fucking reality??? You sick fuck go away!'
And game lets me choose, touch Leo or not, i decided to not touch...
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 )
1. Yes, correct. I was about Sydney, just his name flew off from my head for some reason.
Histeria huh?
But... still, how the fuck it... works?
I mean, there's some trigger for it? Or what?
The strange part was, when everyone seen 'distorted' Chase, i mean Mitcha explained it, but im not sure if i get it right.
Gila? I readed this name, ten times while reading whole story, so, i actually i have no idea who is 'Gila' - i mean im doubting it
Thanks, for giving me answers, i just finished Jenna route, on both endings, where she stays to flood 'whole' echo.
Aaand... where everyone survives, and successfuly, leaving Echo.
I wish this VN to have a whole Film, where you see everything from every route that would be fucking amazing actually.
Spoilers ahead
Histeria was more likely triggered by Chase and TJ who came to Echo, the fact that they conceal murder was enough to start everything again:
"You are only moving in circles" - (TJ route again, it is the most important route in my opinion)
Chase misinterpreted Sydney when he was "drowning" Toby (Toby changed name after that and is known as TJ), bc of this otter drowned him and have sworn that they will hide the truth -> after that everyone's problems got bigger, especially Chase's (night terrors, sleep paralysis, hallucinations) -> gang is leaving Echo -> a few years later when everyone came back histeria should get started, however in this only route it didn't! Because the game that Syd made up was supposed to reveal otter secret and bc of that histeria wasn't necessary -> Alas, in the last moment Chase got possessed again and when the truth was finally revealed to Flynn, otter *sob* killed gila to hide his sin. Chase made full circle, now he will leave Echo and hide to the end of his life the next mystery ;-;
Don't think about being "distorted", it is ghost after all :)
I had no idea who this "Gila" was, so I wrote it in goggle and found that gila is the reptile species = Flynn species, its the same when we call Chase "The otter" or TJ "the lynx"
huh? I don't understand again :/
Syd died only once in 2003, after that everything is going straight to 2015 and this year the timeline is splitted because of Chase whose choices define future (Players choices actually xd)
->Carl route
-> Leo route
TSR -> Syd death ->Jenna route
->TJ route
->Flynn route
Spoilers ahead
There is scene in Flynn route where we see grown up Sydney, however it is only gila's visions before he die in mines. It is not confirmed, maybe it was another dimension/timeline where Syd wasn't killed by Chase and when he became socket-monster he started seeing those timelines? idk