I'll have a go. Adventure Time takes a more absurdist approach to humour and story-telling.

It also reminded me of "Scherzo", which also involves the Doctor and Charley being blind, only they have to evolve forwards instead of backwards to survive the horror.

Yes. Yes!!!
This man speaks the truth!

I don't think this is the first time Rose has appeared human. There were some fun bits where she's messing about with Greg in "Lion 3: Straight to Video", like fumbling about with the video recorder.

I don't think the implication of this episode was that BMO is evil. She was more like an excited kid having fun playing Kung Fu. Not so much "yay, I murdered someone", as, "hey, look what I can do."