Are convergently-encrypted ciphertexts safe from collision attacks?
The answer should be "No. A convergently-encrypted ciphertext is not safe from collision attacks."

Just because it is encrypted convergently does not mean it is not subject to collision attacks. MD5 has 128 bits, therefore, I could expect to have to generate at worst around 2^64 attempts to crack it.

Each collision attempt is more complex to generate because of the need to generate a new key via SHA256, reencrypt, and recompute MD5. There is no need to re-SHA256 the entire plaintext each time for the key, and it is also not obvious whether or not there might be a way to exploit known weaknesses of the MD5 hash to crack it in less than 2^64 attempts, but the question as stated was whether it was safe from a collision attack, and the simple answer has to be "No".

The 2^64 worst case seems to be similar to that of the Google SHAttered SHA1 collision demonstration even if generating each attempt requires more computation for convergent encryption. "Safe" might be a relative term that I did not define. I will try to be more concise in the future. But using MD5 to guarantee non-colliding convergently-encrypted content should not pass a security review. This would be why in the SHA2 specification there is no hash with less than 224 bits, which is 96 rounds greater than MD5, a factor of 2^48 difference for collision attacks.