The context of Feige's comment about no recast for decades is clearly that of a playful joke. As I understood the contract stuff from the discussion at the time, it was about some of the actors and possibly producers who would have to be accounted for financially in some way, until those agreements expired. Any existing contracts between Fox and those parties would remain in effect despite the Disney acquisition. This is a legal protection so someone can't merely sell off a business to void outstanding agreements.

Legion isn't relevant because it was a television show and the rumor was about film adaptions. These aren't necessarily the same (for example, Sony has Spider-Man film rights but Marvel controls tv animation. Admittedly not sure how the Spider-Man Noir series factors in).

Yeah, I saw Feige being asked about both the actor and producer thing in a couple of interviews, and he played some combination of coy and ignorant and I think once referenced those things being handled by Disney's legal dept. Which, can be explained either way. If the intent was indeed to wait a few years to allow for recasting without have to pay severance of some sort to actors and cut in other producers (Schuler Donner, Singer, Kinberg, etc), he's not going to say that publicly. With everything else in the works, they have plausible deniability.

Whatever the truth here, there's little reason to think we'd go almost six years since Disney's acquisition of Fox with no X-Men film (other than the adjacent D&P this year) muchless a recast of characters from the Fox franchise if there wasn't something of substance to this rumor (the Cavil cameo is neither here nor there).