PM'd the results, as they were too long to share here.
On a side-note, given that I'm the only one who seems to have experienced these issues (I keep hoping for my sanity that someone else chimes in with a "me too!") I'm starting to wonder if it's specific to my setup (RPi-0, running DietPi).
I may go with a clean install of Raspbian Lite to see if the issue persists, as I'd hate to take up too much developer time if I'm all alone!
It is worth mentioning though that everything was working flawlessly in v4, this issue is in some way related to the beta as far as I can tell.