Your post wasn't addressed to me, but I wanted to comment on it. We do know from the original films that a second Cerebro is built in the mansion and it seems logical that Magneto's powers would be needed in the construction of that big round room with all the metal panels.
You're right that we haven't seen it yet, though.
In a sense, both you and Dagenspear are right. You are right that there are unanswered questions, but it's also fair to say that some of the suggestions for answering them make sense. Fans of a franchise are always going deep into it and filling in the blanks themselves - this happens with all films and certainly all the films I see discussed on SHH. I've seen people coming up with explanations for the unanswered questions (and/or plotholes) in every single film debated on here. The Dark Knight Rises, Superman Returns, Looper, Avengers, you name it and it's been discussed and dissected to death!
Hopefully, Fox's attempts to create a shared universe prevents these questions, inconsistencies and continuity issues from occurring.
Yes, we don't know at this point how FF will feature in the shared universe. I'd imagine that they first need to be relaunched with a new origin story and new cast, before any crossovers occur. So I would think we won't see them meeting each other for a while yet, but I have no idea whether that's right.
Of course, Franklin Richards is part of DoFP in the comics, so it's possible he could appear there (but it would be a bit meaningless before we see his parents on screen first). It took six films before Avengers so it might take six films from now until an X-Men/Fantastic Four crossover (which would probably see them facing off against Galactus).
They could also have included Daredevil if they'd got their act together in time. Joe Carnahan's planned 70s-set film for Daredevil could have fitted in with a 70s era X-Men First Class threequel - maybe the newly-blinded Matt Murdoch, struggling to make sense of his new abilities, first went to see a specialist in superhuman powers (Xavier? Beast?). And Daredevil could easily have existed alongside Deadpool or Wolverine on screen. There would have been potential there. The danger is in making it feel forced, but then again all these heroes have met up and crossed over in the comics.