Spoilers alert!
I apologize for the long post in advance.
I recently finished all the routes in the game + Route 65 and agree with most of what you said. I haven't played The Smoke Room yet, but have seen Build 2 for Arches, so I may be missing something talked about in TSR.
1. Chase does appear to be possessed, however, I'm not sure that Sam is actually good. In Flynn's route he pushes Chase and Flynn to condemn each other and guides Flynn into the mines to become the Red Socket Monster. Maybe I misunderstand Sam's reasoning, but between that, his constant insults towards Chase, and how Mr. Bronson and Sydney reacted to being possessed by Sam, I'm led to believe that he is probably not a good spirit either.
2. I think you are right on this one. Flynn never really explains why he knew in Leo's route and its not explained in other routes. I assume it was either the Hysteria giving him that info (Similar to how he was led to the mines on his route) or that the Moore family knows more about the source of the Hysteria and Flynn's aunt tipped him off.
3. I 100% agree with you that it is likely that Chase was possessed when killing Sydney (if that is what happened). Chase is repeatedly described as being a good simmer for his friend group but bad by Otter standards. Sydney (also an Otter) is more athletic that Chase but somehow could not hold his own against Chase. While Chase did get a surprise attack on Sydney, the speed and power needed to hold down another person long enough to drown them is beyond a 10 year old in my opinion, especially when they should be able to hold their breath as long as you. I think Chase was given extra strength to kill Sydney by the Mirror Ghost (Likely a spirit that wants the Hysteria to occur) to kill Sydney. This would be similar to when Chase gains strength to beat up Flynn in TJ's route.
I do want to mention though it is strange to me that the only route that attempts to confirm Chase killed Sydney is TJ's. It is touched on with people accusing Chase in Leo, Flynn and Jenna's routes (And I assume Duke was accusing Chase in Carl's route, but it never fully came up), however, none of those routes fully confirm it. Chase when not possessed (by the Mirror Ghost) doesn't seem to really remember it, but in TJ's route he gets either fully or near fully possessed and seems to realize what happened. Only in TJ's route does he seem to admit to killing Sydney. I have some theories regarding the Timeline and how Route 65 works as a prequel but this is already a long post.
4. I think you got this dead right, so I have nothing to add.
5. So the Hysteria seems to be a cyclical aspect of Echo, caused either by the Quartz in the mines or just some cursed aspect of the land itself. The area absorbs the negative energy of the people who are in the area over time. Every so often the negative energy is released back onto the town's inhabitants and you get a Hysteria Event. The trigger of an event appears to be whenever a murder occurs and the killer gets away without being caught, but I think that is not quite on the money. For example Mr. Bronson and Janice killed Sam and hid the body, but there was no Hysteria in the 70s (The most recent event at that time was the 60s if I recall correctly), Sydney's accident with his father could also have been a trigger but nothing beyond the possession seems to have happened for either of them. There is also the issue with Brian killing 19 people and none of their deaths appear to act as a trigger for the Hysteria, but somehow Chase returning to town is the trigger. It's also mentioned that the Hysteria would have happened eventually even if Chase never returned, so Chase's return was just the tipping point. This leads me to believe that the true Trigger is Chase having relationship issues with the rest of the friend group.
Each friend has some reason they care deeply about Chase and want him to spend time with them, so when Chase focuses on someone else their issues seem to intensify. Examples include: Leo's Chase Tulpa, Carl's route being about a haunted house which was why Chase spent the entire summer in 5th grade with him as well as Carl having an imaginary Chase he spoke to when crashing his car, TJ always wants Chase to join him on a hike and Jenna mentions that TJ was most excited to see Chase, Flynn has a strong attraction to Chase but believes that it wouldn't work due to a variety of circumstances, and last but not least is Jenna mentioning she gets jealous of what she can't have and when Chase spends time with her she flaunts it in front of Leo. I think that due to everyone wanting Chase to spend time with them, stresses the group and also may be the reason why Chase appears to be the center of focus for the Hysteria. Throw in that Chase probably killed Sydney on top of that and you've got a huge storm coming, honey.
6. Carl's route is a bit off, but that may be due to it being written first. James and John are possessing their descendants (Carl and Jenna) in a fight to either hide what really happened or reveal the truth of who was the pedophilic serial killer (The worst combo). Chase gets wrapped up in this due to the town using him as the witness that has to figure out which spirit is telling the truth and which one lies, while Raven kinda got stuck by being in the wrong place at the wrong time. Carl's route feels the most disconnected with what is going on in Echo, but that is due to it really being focused on what happened around the time of The Smoke Room. Since it takes place around that time it makes sense for TSR to have a few references to Carl's route.
As stated before I just finished the game and haven't check out TSR so I could be missing somethings and this is all just my two cents. If there is something wrong or you have a different theory, I would love to hear it.
Thanks for reading my TED talk.