I highly recomend you either replay the game or look back on scenes that confuse you or both because that shit helped me a lot and i recommend you try to come up with your own conclusions as well.
1. the thing with TJ is MOSTLY ambiguous wether or not he remeber what happened or not since the game is well know for letting the reader put the peices together themselves, I think he doesn't remember what happened to sydney but he knows the version of it he told wasnt true. In flynn and (especially)Leo's route after he almost drowns and chase saves him (which he doesn't in flynns route) he begins remembering what happened and starts looking at chase weird. In flynns route his reason why he didn't tell what he actually remembers was because he saw the socketman ( thats what the fanbase calls the monster) and he did tell people what happened but no one beleived him because of him seeing this monster so he never bothers. In TJ's route I think an incident that happens to TJ off screen that not only causes him to make the scavenger hunt but also seems to make him remember what happen (most likely a possesion of some sort).
2. so the whole flynn thing is still tough the explain, the gist of it is that flynns going through what could be interpreted as time travel or a eternal vision/limbo like state that isn't actually real or a combanation of both where he's seeing all of these fucked up things that happened in the town and the significance of this is his desire to know what happened gets taken advantage of and he get into this situation where he's now able to see everything about the horrors of echo and esentially becomes appart of the hystaria in the town but is denied seeing what actually happened to sydney, the whole reason why he went this far in the first place. I say it's tough to explain because both possibilitys of flynn being or not being socketman have contradictions and it's intentionly not supose to be fully understood since even flynn himself has no clue what's going on but just goes with the ride hoping to get to the truth.
3. socket mans motive is never understood or mentioned if it has one. I think that socket man is a amalgamation of all or most the people who have ties with echo and died in the town (flynn becoming one of them at the end of his route) and some times random people control it. What seems to be the biggest idea with the fans is that every time the gang gets there asses saved by socketman is that it's flynn from the end his route.