Hi!
Regarding removing older versions, the DevSite doesn’t currently support a way to do this. I can bring this feedback to the team to be aware of this ask.
Additionally, for the Extension Manager, we have received feedback on the issue with providing no high-level versioning outside of the detail pages. We understand that this is causing overhead on developers as they add versions and are looking at what we can do to incorporate a version number on an Extension Card for developers in the short term.