SHA512 vs HKDF key expansion
I'm not aware of any attacks on SHA-512 this way. I would create a small function to validate that the input size to SHA-512 is indeed identical to the seed size though, just in case. Even without that the function should be secure. Kind of related is my question about KDF1 and KDF2.

Note that implementation of HKDF-expand from a hash should be pretty easy, especially if a HMAC construction is already available. KDF1 or KDF2 would be even easier, it would probably be trickier to find test vectors for those KDF's though. So you might ask yourself why not to use HKDF-expand.



Generating a 256 bit key using a KDF over a 128 bit means that the security margin would remain at 128 bit. Basically it's fine as long as you don't claim a security margin of 256 bit. Just claiming AES-256 bit encryption would probably mean the same thing to most people.