Hi Typical_Name! Thanks for your comments!
The Panacea issue is a problem with the item description accuracy rather than a bug specifically. I will try to address that in a future update, but the gist of it is that Curse is supposed to be the "ultimate" debuff that only an Elixir can remove, yet that caveat got left off of some item and skill descriptions. A Curse spell is available to the player as well - but is well hidden - and causes the targets to take additional damage, while lowering many of their attributes significantly.
Defeating the final boss unlocks a specific feature that can be brought into a new run. I don't want to give too much away, but that information is crucial to seeing the "true" ending and breaking the cycle.
As far as the upgrade system goes, there is a bug/exploit that is possible to achieve with the scenario you mentioned. It's sort of a technical limitation with the engine, though I don't know if I would patch it out regardless as some players really dig those sorts of things. I'm glad you like the Mana Bolt skill! I think it flies under the radar a lot but is quite effective and supports a lot of builds. (that said, once you unlock a few specific upgrades I think you might replace it eventually :) )