ATJ has already said he wasn't going to reprise the Quicksilver role. So I can see why some people automatically went to Peters coming in to reprise the role, but just doesn't make sense. I do agree like I mentioned before that Speed makes the most sense given that Young Avengers is currently being setup, especially since Teyonah Parris has been cast as Monica Rambeau who is also part of the Young Avengers with Wiccan and Speed.
As for the secrets, look at Season 2 of Mando, there is information leaking like crazy. So with WandaVision other casting news has been leaked, like the Monica Rambeau casting.
And no I don't expect Feige to bring in the FoxMarvel versions of the X-Men to be brought into the MCU at this point. I had thought he might previously, but that was before Dark Phoenix. He wouldn't want the negative baggage to be brought into his successful MCU franchise.