Ciao. Thanks for your patience.
I found the time to go through your log entries.
I see that the problem seems to be only in entries for your tests, regular sales are fine. I suspect there is something wrong with the new shortcode you're trying, so went looking for one of the pages you linked to, and looked in the editor. Sure enough, there's a problem there. Here's a screenshot, there are a couple of
in the code, probably added by your editor.
I would fix the shortcode and test again.
About demoted users paying and not getting upgraded back, I suspect it's the login session. Are they logged into their account when they upgrade again? It's best to have an upgrade form in a page protected at level 0, that way only logged in users can access it. This ensures that their existing account gets referenced and upgraded, instead of the payment going towards a new signup.
Please test those and let me know how it went. I look forward to your update.