I'm confused about the idea of casting multiple actors in the same role as a solid basis for a multiverse because my experience reading multiverse stories, particularly DC ones, is that the appearance of the superhero doesn't actually change in the multiverse unless the superhero is an entirely different version of the character (e.g. Kal-El vs. Calvin Ellis).
In multiverse stories, the different versions of Superman, Batman, Wonder Woman, etc. don't look different; they look the same. These characters only look different if they have completely different backgrounds and origin stories (e.g. Diana vs. Nubia). Spider-Man: Into the Spider-verse follows this trend: there are similar looking Peter Parkers in the multiverse, but there are also different looking Spider-like heroes (e.g. Miles Morales, Spider-Ham).
In other words, a multiverse wouldn't feature different actors portraying the same character unless they weren't the same character. A multiverse wouldn't, for example, have a biracial Arthur Curry and a white Arthur Curry or a Batfleck and a Battinson. Spider-Man: Into the Spider-verse did not portray Peter Parker as someone who looked different throughout the multiverse.