I do remember what Will's stated purpose is for locking up Barnabas, later. We see it here too, with Will talking about how Barnabas will be his salvation, or something. Nothing about that last scene indicates that he has saving Carolyn in mind; it's just that that would make more sense than what the purpose turned out to be. I thought watching that scene: "Why don't you just ask him nicely, Will? [spoiler]Why wouldn't Barnabas help you with your research?[/spoiler]" Then I thought, "Oh yeah, Carolyn's a vamp victim too, he may need to stop that ASAP..." Plus there's his own safety.
DS rarely had any patience for acknowledging or dealing with the basics of everyday survival, like toilets, or money. Mostly that's a good thing, but too many characters are now landing in town with no means of support and no clear place where they live, no hint of who does the meals or cleaning up, etc.. It gets especially implausible when Barnabas gets to continue operating as a wealthy member of a wealthy family, no matter where he goes, no matter who he's posing as. It's as if having money is just intrinsic to him, just his basic nature, and it's true wherever he goes, because that's who/what he is. He could find himself in the Stone Age, and he could still live on his undefined, offscreen money. And they say we don't have a class system in the US...