Rem made multiple copies of his tracksuit but how they still exist after her name was eaten I guess doesn’t really make sense.

Since just her name (and memories) was eaten, she still exists as does all the things she’s done. Just everyone’s memory of her is gone. It’s different than when the white whale ate her out of existence.

As for everyone else, it’s just that typical anime thing of keeping characters in the same outfit so you don’t need a design sheet for multiple outfits.

This is the most correct answer. But hey, we did get yukata recently! Also in an anime with a lot of supporting cast, keeping them in the same outfit helps prevent the viewer from going “wait, who is that character?”