Holy wall of text.
Anyways, lemme try to answer this step by step.
I'll check out why Waschbar Ritual spell isn't working as expected. I had to fix a bug related to it previous update, which might have broken the second function of it.
Priscila really needs some extra skills to make her more reliable. I thought about adding more dance moves to her, once I think about what they could do.
The Potion Throw skill actually increases the effect of the potion thrown at a character too (they heal more), alongside adding an extra chance of healing a character (Since Potion Throw doesn't use potions cooldown). Due to Benny making it easier to get golds, he can buy strong potions to be thrown in battle when in peril.
Buff stacking can happen if they're from different sources. The same is also valid for debuffs.
Btw, if you have Discord, I recommend accessing the game discord server to send feedbacks and bug reports. You can find the link on the download page of the game.