That's a loooot of speculation. Cable, who has lots of experience with them from the future, seemed quite certain the EMP grenade would do the trick....
Jean explains his age during her press conference. She said Charles was 11 when he started hearing people's thoughts in his head. And then continues...
Agreed, this Sinister is so effective. Hopefully the comics will shift back to this one. I can't imagine many people still want Gillen/Hickman/etc's...
Mercury was probably resurrected off-screen at some point, given how Onslaught attached to people who were resurrected to manipulate them, and we saw...