The code uses CFB mode, so fixing the IV results in a constant first block, which gets XOR'd with the first plaintext blocks to make the first ciphertext blocks. So knowing one plaintext gives you access to all first plaintext blocks.

One problem with your proposal is that for CPA security cuts the security level of the scheme in half. For an n-bit hash about 2^(n/2) operations are required to find a collision, and given a collision you break CPA security.