This is deep, and utterly persuasive. I would only add that, in a textually corrupt passage in which Shakespeare accidentally lets his source material show through, Mole/Hamlet actually mistakes Polonius for the Rat - which suggests that he expects either R or G to be hiding behind the arras.

I like your R&G theory, but to complicate matters further I would suggest there is also a smidgeon of Rat in Horatio - whose name, after all, is simply Rat with a few trifling letters on either side.

(Yes, I unrelatedly am a great admirer of Prof. Crews' work, both the original and his much later sequel, Postmodern Pooh.)