Been working my way through the B&Ms - they're all good but this one was by far my favourite, I think the length was justified.
Regarding M's homosexuality and attraction to Castor, doesn't he say, in the interview at the sanitarium, "Like you, I am what I am"? At that point, the reader/listener thinks M means he is a servant of Crown & Church and, as such he views Castor''s sexuality as an abomination but what he really means is "I'M GAY TOO!" [EDIT] not that that means he should be automatically attracted to him, of course, but as someone said up above, the description of Castor does suggest that! [EDIT OVER]
I like the fact that we are gradually learning more about this pair with each episode, I think an "origin story" story would spoil things!