All done! Once again, my degree in Classics was of limited use: only one of the mythological pair was familiar to me. Thanks to wikipedia, I found the other half and also the other halves of three other thematic names I spotted.
It looked quite tricky initially, but after cracking 16ac, it became easy - even with my rudimentary knowledge of the "six-letter adjective".
After seeing the last three changed words in the down clues, I spent a while wondering if there was a hidden message to be found by juggling ALL of them.
Lovely, educational puzzle, Lionheart, but I think I'll wait for someone else to decipher the title.