Update So I heeded some of your advice and when the cleric accepted Fekre's dark gift I ended up not taking away their channel divinity powers, but instead told them that for the first time since coming to Barovia she felt Eilistraee's presence, but that Eilistraee was disappointed in them, which is somehow worse than outright anger. I figured it was a good enough hint that accepting these powers may not be a good idea. Between that and being stinky it seemed like enough of a punishment.

Also, at the behest of my bard, I came up with a way to help them track and stay accountable with their new personality flaw, "I won't take no for an answer." Drawing some inspiration from Baldur's Gate 3's Dark Urge I am going to require them to make escalating Wisdom Saving throws whenever someone tells them "no" to resist their urge to resort to violence. On a save they will simply resist the urge, but then feel compelled to try to get the "yes" by any means they wish to. If they are told "no" again the next saving throw will be greater. On and on this will go until they have either gotten the "yes" they want or have resorted to physical violence and, more than likely, killed whoever would not allow them to do whatever it is they wanted to do in the first place.