Yeah, I thought they traded places at the beginning of the final monologue. I'm still slightly annoyed that Breathing technique pedigree is still being ignored (as in they won't explain how he learned Mist Breathing) but I think my expectations are slowly aligning now to the fact that it won't be consistently happening.
The whole "mark" thing is also inconsistent now. At first it looked like Tanjiro got it due to his use of First Breathing, but Muichiro has developed something "similar" through gaining his memory, guts, talent and resumably being a descendant of the First Swordsman - but he does not perform First Breathing as far as we know.
I'm also enjoying Kanroji's eyecandy a lot more in the OP/ED than I thought I would.