it's not about whether or not those people can be bothered to contact you and 'claim' ownership of their work, it's about the legality of you claiming copyright on their work up front. if you are taking the launch of this studio seriously you might want to do some research on the legal ramifications of the liberties you are currently taking with other people's work.
imo, continue with PMs, personally, if the artists where this bothered they wouldnt have contributed anything in the first place, and dys assets wont, and probably cant be used for a commercial purppose.
dystopia was copyrighted by the dystopia team, which are now the punyhuman team? if someone contributes and/or leaves the team, what difference does that make?
(i have no idea btw, its just a question, any lawyers in the house?)