You could do (a) to force active attacks, and then at the end (after SNI) authenticate the server based on the then-matching cert, followed by checking that you

You could do (a) to force active attacks, and then at the end (after SNI) authenticate the server based on the then-matching cert, followed by checking that you both got the same symmetric key from the ECDH key exchange at the beginning. If they don't match, you know there was an active MITM and both sides know.