No, they should definitely not be listed if they are only mentioned. The rule is that a character must appear in the comic to get an appearance credit. It's called appearance for a reason, so a mention, a photograph or a wax sculpture does not count as appearances.
Nor does someone like Mystique shape-shifting into someone else. Only Mystique would get the appearance, not whoever she shape-shifts into.
There is no rules about who gets to edit what, the wiki is a cooperative effort, not a competitive. Having that said it is a bit annoying when someone edits the page you just added, so it is sort of an unwritten rule to wait a bit before editing. There are lots of comics that doesn't get added and lots of pages without any info, so i don't think it should be a problem to find other stuff to work on.