] Hamlet has known them all his life, but they were "sent for" -- from where? If Hamlet's always known them, wouldn't they live where he grew up?

As far as I can figure it, and I really don't know why I thought this, but I have always thought they were in Wittenberg. In the very beginning of the play, Hamlet was asked to stay at home and not go back to school. My guess is that R&G have been at school. The only proof I have for this is Gertrude's line: Good gentlemen, he hath much talk'd of you; as if she had never met them, only heard of them. From that I guess my mind lept to them being childhood school buddies. Not that this is textual proof, many times I have seen their appearance rationalized this way, often with Gertrude not knowing one from the other.

] So why is it that they cannot figure out anything whatever about what's going on?

To start with they are briefed by Claudius right from the start. They seem to be htere in an effort to honestly help an old friend. In their minds, I suppose it isn't inconceivable for them to think, How can Hamlet be upset with the marriage if his uncle and mother seem to be so very concerned about him? Also, Hamlet is onto them so quickly, he doesn't let them in at all. He does nothing more than exchange quips and then immediately accuses them.