Having watched WandaVision, I can't help but wonder whether the title for this film is misleading.
When I hear multiverse, I expect that to be parallel worlds and/or universes; that's how and where many people expect the x-men, f4 and Deadpool to appear.
Knowing also that despite speculation, hope, desire and rumour, we got no multiverse ;of that context) in WandaVision, and the one character we thought who was ... wasn't.
Therefore, all this talk of Toby & Andrew as Spider-Men ... I'm seeing them as Spider-Man fans within the MCU who, for reasons yet known, important to the plot of the films (Strange 2 & Spider-Man 3) but not as actual multiverse counterparts.
As far as the X-Men, F4 and Deadpool go, the only legitimate way I can see that taking place is through some dimensional crack; similar to DC's flashpoint.
Reality as we know it will change (we already know Wanda can do this to some degree), so it stands to reason that reality can be changed on a much larger scale.
As such I'm going to propose the notion that once this multiverse arc comes full circle, the likes of the X-Men, F4 and Deadpool will always have existed in the MCU, and only certain characters (Wanda & Stephen for two) will remember the old reality.