At the end of the movie, Columbo is about to hit the victims daugther in the face after she has tried to do the same with him.
Is this just an act between them in front of the killer, or is he really mad?

Good question. I have the impression that this was not an act. Columbo really was angry with Margaret. Keep in mind that it was only the second episode and his character was still in development.

They reconciled shortly afterward and Margaret helped Columbo trap the killer. (Just as Joan Hudson came around and helped him trap the killer in "Prescription: Murder")

Correct, that part wasn't staged, because while they shared similar suspicions and discussed the keys in particular at the restaurant, she acted on her own.

The arrangement came later -- over the cash.