Dexter is all fucked up about Deb. So yeah, in a way, he is incapable of critical thinking. The thing that kept it from being horrible writing is that he realizes it at the end of the episode.
I'm not saying it's nobel prize in literature quality -- but it was cohesive and explained itself.
I don't think there's going to be any huge twists this final season. There might be one big one (like the shrink lady, in a surprising twist, actually ISN'T the killer!) and another small one, but so far this story is more straightforward than the show has been in 3 seasons. It's a real straight Dexter redeems himself (somehow) through Deborah story.
S1 started with his real brother and his fake father. S8 ends with his fake mother and his (real) sister. There's a symmetry to it.
The biggest possible twist is that Dexter doesn't redeem himself at all, and the show ends in some amoral nihilistic wasteland. Which would also be acceptable.