This started out so easily! I'm still none too sure about Les petits cochons. Is the letter-change really the second letter of the second word (rubbish!). Or have I got a dirty mind? I'm sure I have 12A and 15A, but can't parse them (especially 15A). And I have to change two letters, the first and third, to make the first half of 5D into the second. And there is a river, probably mutated in 7D, I assume.