MAJOR SPOILERS:
So my take away from Flynns ending other than it being extremely sad is that Flynn doesn't become socket face that appears in other routes. It's more of a vague time traveling in the sense that he's experiencing events that the socket face was there for whether it's some state of limbo or Flynn is actually there is kept ambiguous and could most likely be both in some scenarios. What leads me to believe this is that you are given the choice to kill Salem, brain, or Keith. The thing is brain is still alive in echo so is Salem until the very end and dies in a different but similar way so it doesn't make sense for there to be an option for you to have a choice here. Along with how weirdly dream like that scenario was with the tv and salem acting funny but what we do know is that this has happened in before in some way, keith did die other wise Flynn wouldn't have any reason to be seeing this it has to be some kind of created dreamlike scenario that has some truth to it. I also notice how Flynns transformation was inconsistent, Tj describes the monster as "3 holes, 2 for the eyes, 1 for the mouth" (flynn also has control in this scene) but flynn's eyes pop when he tries to see what happend to Sydney which was after tj saw socket face and he's blinded but later is able to see a younger jenna. Speaking of TJ flynns route is only one where he mentions he saw socket face and recognizes it in the car scene "its real!?" but in Leos route he doesn't even recognize it "what was that!? what was that!?". The ending is really confusing and ambiguous for a reason so it's fine to come up with your own conclusions.