I don't find it so odd that Nathan tries to talk Trask out of going to the Old House. Trask has promised to help Nathan in his career in exchange for his perjury, so I suppose Nathan has some vested interest in keeping him alive. And who knows, Nathan may have some tiny trace of good left in him, or at least a nagging conscience.
After Nathan finds out from Trask that Millicent looked as if she wanted to stop Naomi from tearing up his (N.'s) letter, he warns Trask one more time, You must not go to the Old House! Then Nathan leaves, never to see Trask again.
Barnabas tells Ben, "I am planning some entertainment for my guest, to be followed by a "reward" for his treatment of Victoria Winters." The "entertainment" line is pretty good too. Ben looks sick with dread, but Barnabas clearly is enjoying himself. For some reason, he's not wearing the famous onyx ring, but maybe he just didn't want to get mortar on it. From this point on, Ben has no part in the proceedings, and we have to assume he's off drowning his sorrows somewhere.
After Nathan leaves, Trask hears Abigail's summoning voice. Come to the Old House, she tells him again. He picks up a candle and looks around the room fruitlessly for the source of the voice. I will obey your summons, he calls out. Setting the still-burning candle down, he leaves his room at the Three Bells for the last time. After he does, some unseen agency blows the candle out.
Yes, how DID Trask find out that Barnabas had died? Did the doctor blab? That doesn't seem likely, though, especially since Barnabas supposedly died of a plague.
I think Barnabas was so bent on revenge for Vicki that he didn't want to waste time trying to persuade Trask that Angelique was the witch. I think it might have been more effective for him to show Trask some incontrovertible proof (I don't know what) and break his spirit. But I suppose DC couldn't resist the opportunity to steal yet another plot (from "The Cask of Amontillado").
When I first saw this ep., I thought it was pretty strong stuff for daytime TV.