Opening Michael's grave was accomplished under calm skies, and Julia was dry when she arrived at Collinwood to force an explanation out of Barnabas. The two of them decided to head for the privacy of the Old House, the DS theme played, and then cue the thunder and lightning, the mood lighting, and the raging fire. But that's not the most implausible part...
I get why they added the never-before-seen part with Oberon. It served to impart information that had until now unfolded slowly over nearly 50 eps-- from the start of the storyline right up to the last one. In other words, it was an easy (perhaps lazy) way to insure that all viewers got up to speed on the story. But I happen to think that Leviathans, in theory, was not a bad idea, so maybe I'm alone in thinking that catching up this way is the same as skipping chapters of a marvelous book and reading the Cliffs Notes instead. But even worse, to insert the scene at a point when we thought Barnabas was unconscious threw the door open to inconsistencies, as retconning often does. For example, it makes no sense for Oberon to tell Barnabas the details of the punishments that the Leviathans had in mind for him if the intent was that he would forget them anyway. And it clearly was not the only thing that Barnabas didn't remember about Oberon's lesson; based on #915 alone, Barn came to the present not only unaware of the background of this race of people (Adlar had to spell it out for him even though Oberon covers it here), but he seemed clueless that any punishment awaited him at all and horrified that the Leviathans knew his dirty little vampiric secrets (despite Oberon telling him here that "we know everything about you").
Like Patti, I hated the revelation that Barnabas was drugged at the cairn. No no no! What need would powerful, ancient beings have of pharmaceuticals?
BTW, the hand of Sheriff Davenport belongs to Assoc. Prod. George DiCenzo.