I solved the failing login to plasma with help from Arch forum 
But I cannot post links yet. In short:
I’ve tracked down the json problem to ~/.local/share/mime/packages and ~/.local/share/mime/mime.cache. For some reason putting either of these back into my home folder instantly breaks everything. I’m not, however, sure if this is the cause of all of my problems. It is a good start, though. I’m not seeing anything broken by that file and directory not being there so I’m just going to ignore that for now. I also managed to rebuild my mime.cache with “mkdir ~/.local/share/mime/packages && update-mime-database ~/.local/share/mime”