My use case is slightly different. I like to be able to log into a
basic user configuration to test something, without logging out of my
current session. That second desktop might even be running Gnome rather
than KDE. Would that also count as a blocker?

Hello Patrick. Our criteria are mostly concerned with a default Fedora installation (Workstation, KDE, Server, etc). We might even state it explicitly in this criterion. So no, switching users between different desktop environments is unlikely to be release blocking.