Hi Dirk, I did so, expecting a hard landing into a grim reality where nothing was as I assumed it to be. But in fact, I loosened them again, as your detailed post broadly chimes with my own experience. Seriously impressed by your record keeping btw!
There were threads related to some of these anomalies, and I posted in one of these - mainly because some of my Qobuz purchases seemed not to make it through to Roon for reasons including the ones you describe.
Still, over 90 percent of my purchases did show up in Roon problem-free, and I could stream all my purchases in Qobuz’ own app, last time I checked, and in the correct resolution. So while I hope that everything I can stream in Qobuz I will eventually be able to get at using Roon, easily and without download trickery, the worms are likely real. But I don’t want to keep a lid on that can! 
I was able to see 90+ % of my purchases in one convenient view… currently I am back to 0. Just don’t think it is an improvement.
(My current workaround to easily get to an overview of my purchases is manual tagging of Qobuz-purchased downloads. For a while I sought to ‘abuse’ Qobuz favourites by adding my purchases, but it is not what I would want to use favourites for, and it also typically failed - in Qobuz’ own app - for exactly the albums that were problematic for Roon)