hashing and message input size If a hash function like Whirlpool or Sha512 would take a byte array of 1 MB in size filled with the output of a Secure RNG as its input message, would this be considered a strong input message, a strong key, or neither ?
Neither.

There is no such a thing as a strong input message to a secure hash function.

Hash functions don't take a key as input. Keys could be used as input to a KBKDF such as HKDF to derive additional keys key secrets. Those algorithms are often build from hashes though. 

The output of a secure random bit generator doesn't need to be hashed to be secure. Many PRNGs / DRBGs are actually build from secure hash functions. Adding a single hash won't help.