Okay so in regards to the Katherine “going to hell” thing instead of the other side, how implausible is that really? I mean, I understand that the other side is supposed to be a supernatural purgatory and to keep them from moving on. Now, this might just be the writers leaving plot holes galore again, but wasn’t there that time when Anna came back as a ghost and said that she was afraid that her mother had already passed on and found peace because she couldn’t find her on the other side? And then there was that point where when Elena asked Esther about Jenna being on the other side and Esther told her that Jenna wasn’t there and had found peace. So if these aren’t just plot holes because we now have the Qetsiyah-creating-the-other-side-to-trap-Silas story, it shows that supernaturals CAN move on after dying.
The way I interpreted Katherine not being able to pass through Bonnie to get to the other side was “Nature” not allowing her to, much in the same way that Nature created the doppelganger line to make up for Silas and Amara becoming immortals. I assumed it was Nature not giving Katherine the chance to find peace because she had been so evil and vindictive and had damaged so many people throughout her life and had caused things to go so out of whack.