And surely "Rat" is German for "advice" or "counsel"?

It is indeed, which befits the role of Rat/Horatio as Mole/Hamlet's friend and adviser. Now I look afresh, I also see that the positioning of Ho and Io ("Io Hymen!") either side of his name is reflective of the whore/virgin dichotomy that Hamlet falls victim to in his view of Ophelia, and that Grahame (who famously boasted that his book was "clean of the clash of sex") in turn projected wistfully on to his gaggle of confirmed bachelors.