So, last night I dug around and dug around. Did you know that Steam/TFP creates a file called "prefs" under ~/home/user/.config/unity3d/'The Fun Pimps'/'7 Days to Die'/ ? Deleting the file, it doesn't get recreated on re-install of the game. Neither does it get recreated on a files validation. Not sure if that's in any way related, but seemed interesting.
Found other bits and pieces in /.local and /.config and even under wine's structure. Though I'm not sure if that's from older usage, or attempts to use, Proton.
Manjaro being a rolling release and this system being effectively 6 or 7 years old (though updated hardware several times), there's a lot of detritus laying around. So, in the end, I uninstalled Steam and removed any references to it that I could find. Then re-installed Steam and the game and Yay!, it works. At least in GLCore.
The upgraded hardware (video card, mobo, ram all replaced in the last month) makes it seem fast even with the lack of optimizations.. well, mostly. Horde nights still tend towards a slideshow on a high-ish pop server.
I suspect my issues came from a regular attempt to see if TFP had fixed anything on the proton side. It seems something gets left on the system while switching back and forth that eventually causes an issue. That's my guess, but I don't have the patience to try to reproduce or try variants to gather evidence. Guess it's just wait and hope they both fix it and put something in release notes. I don't think I ever saw anything in a20 when it started working suddenly there.