Rosencratz protests the King's demand to pump his friend for infomation, yet goes along with the plan in the end. Is there any point to temporary morality? How might other characters become relativistic?
I think Rosencrantz is a bit more simple than that. They are concerned for a friend but easily duped. Maybe it is more of a case of political naivet'e. Claudius plays upon their stops too easily.
I always wondered who played these two for WS. The normal clown role had to fall somewhere. The players and the player King are far too sersious. Maybe these two are more clownish than they come off in a contemporary reading.
To your first question, "Is there any point to temporary morality?", I would have to respond by saying no. For someone to stand up for something they believe to be wrong, then cave, merely shows them (and oftentimes their cause) to be weak and pointless. It also increases the power that the other person has over them because now they see that with a little bit more persuasion, you'll give in and go along with their scheme. If something is bad enough to resist, one needs to be as strong as they can and continue to resist it, despite the pressures. Easy for me to say, but it is what I believe and what I try to teach my own children everyday.
"How might other characters become relativistic?" I can see Gertrude agreeing with Hamlet in her chamber that yes, she will avoid Claudius' bed from now on, because what mother wouldn't go along with their son upon hearing the heartfelt grief and anguish coming from his soul? But will she stick to her guns when Claudius comes a knocking with his "Come hither, Gertie Baby!" looks? Somehow I doubt it because I don't believe she's truly convinced by what Hamlet said, and I also believe she truly loves Claudius and so feels quite torn between the two.