- This also blocks the reinstallation of existing purchases (a very big deal if so)
I've checked this. I was able to install an already-owned R18+ game with no problems. I can't view its store page.
- MA15 games are also blocked (I don’t believe that is true though?)
Not as far as I've seen. The Witcher is MA15+, and I can view its store page.
That's all just my experience, though. Maybe other people are having a different experience, but I suggest people try for themselves instead of accepting worst case scenarios people are saying online.