Ok, I'll copy/paste what I sent to MovieLord. Well, fixed some little mistakes I had made.
Serious spoilers.
Edit: Ah, didn't notice rachetdream's
to be continued after the credits, since I turned off the game.
He also explains it better.
Anyway, I still leave them here if you want them.
[QUOTE=Ghost Rider LSOV]Well, I'm not a great fan of spoiling nice stuff, but if you insist...
1. Who turns out to be the REAL speakeasy arsonist?
Like many others had already guessed, there's only one person who wouldn't like hooligan stuff in her town.
And you learn it quite fast in the episode too.
2. Are Trixie and Sylvia (the person Arthur McFly's SUPPOSED to marry) the same person?
Well, that's one left for almost the last, but yeah, "You didn't think my real name was Trixie Trotter, did you?" (or something like that)
3. What will become of Edna by the end of the game?
Certainly gets a happy life it seems, but you would never expect with whom. :P
With Kid. 
4. Which 2 characters does Michael J. Fox voice in this episode?
William McFly and... quite a few Marty McFlys at the end. :P
and 5. The big one: HOW WILL IT END?
They go back to the sale, but it's a garage sale that Doc is having now (altered timeline) and he says that Clara and him had always a part time residence in Hill Valley "wasn't that how it was when you left?" (Marty to himself) "...no...".
And we learn why did Doc went back to 1931.
He wanted to research all McFlys and give their history as a thank you to Marty for saving him in the movies and so went through time and the one he had troubles with was Sylvia (since she went by the name of Trixie). 
Of course, the way the game ends, I guess he'll have to make more trips in the ...future? 
Edna comes with Einstein and Doc thinks she was doing some harm to him, but she explains that she did what she would do, take Einstein for a walk.
Doc and Marty are getting really weird vibes and Doc says they better go slowly back into the lab, before they see any more changes. Then Kid comes up and calls to her since it appears they really got together while spending time in prison. They then call Biff and leave as a happy family. (weird for Tannens eh?)
And then, a Delorean pops up, a future Marty comes out (Michael J. Fox) with the usual "Doc, you have to come back with me" "Where?" "Back to the Future!" "But if your past self sees you *Marty is seeing him clear as day at that point
* there will be irrepairable damage" (or the usual stuff he says) and then future Marty says that it's ok for our Marty to also come with him. Then another Marty from the future comes with a blue Delorean (MJF again :P) and claims the other Marty is lying, then a 3rd Delorean appears (a black one I think) and a more biker-like Marty (MJf of course) comes out and the 3 Marties start arguing between themselves.
Of course Doc goes all "the timestream is being destroyed" and game Marty asks "but who is the real me?", where Doc just looks at him and smiles, so they go in the Delorean to fix everything and Marty asks the date to go, to which Doc replies "Mr. McFly, thrill me." and like in the end of BTTF (the movie), they go airborne over the road and fly towards the camera. Then the credits come up with the Back in Time song. 
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