Is hashing a list of hashes safe?
What you are describing is essentially the same things as a hash list. A hash list is a sequence of hashes over which another hash is calculated. Your scheme does the same thing after sorting. The sorting won't matter for the security of the scheme; it won't increase the chance of collisions.

Hash lists are also used for a well known structure called a Merkle tree, which allows for hashing data in parallel. Aggregating hashes using another hash is also performed in a meriad of protocols, including PKIX protocols (digital certificates, Cryptographic Message Syntax etc.). In those protocols the final hash is usually used within digital signature generation.