How resilient to attackers with extreme resources available is this encryption method?
All looks pretty secure except for your auth key derivation. You should use a better key derivation method like HKDF instead of just SHA-512.

I don't think your random nonce is doing anything in this scenario - an attacker who wants to brute-force a weak password wouldn't be slowed down by a nonce transmitted in the clear. Why not just use a randomly-generated AES key and a key exchange scheme? This would really confound an attacker with massive (e.g. nation-state level) resources. The NSA would laugh at your puny attempts to use scrypt to slow down brute-forcing.

EDIT: Wikipedia's article on Diffie-Hellman key exchange should get you started.