But to SELL it you’d need the permission of the songwriters and record labels, and probably agree some sort of split on the royalties. But is that actually so unreasonable, given it is their work you’re using? Have you sent them your remixes and asked if you can release them?
No, you can't. You have to procure a license to DJ publicly. A Public Performance License is offered by most of the major music conglomerates, or you have to find a venue that has or will pay for such a license. Profiting off infringement worsens your situation legally, but any copying and distribution by any means of copyrighted material is illegal without a license.
Maybe a while ago it was. It no longer accepts new papers since 2020 and there are much better alternatives available are this point (STC Nexus)
I want dozens of them. And then I can play the one with the best reviews.
Of course, without copyright meaning that people have to pay for the games, the chances of those games being made is pretty slim.