Easiest way, and probably the approach they'll take: there were was always the odd mutant here and there, many possibly already in Krakoa, but for some reason or the other, the number is increasing exponentially in the last few years (the snap could easily be used as a hand-wave, make almost all the new mutants people that died there and were resurrected) meaning the public is becoming aware of their existence.
Magneto not looking like a 90 year old can be explained with either Krakoa drugs that make all mutants not age (and could be something they offer the world), or if they want to go there, he died and was resurrected.
Some of the ideas here (focus on characters with complicated stories like Bishop or Psylocke, have the X-men be retired superheroes, start with time-travel) are just non-sense.