AFIK, HMAC is needed only when you use a non self-authenticate symmetric encryption. For example, CTR mode must be used with external authentication (HMAC). But, in principle, CBC does not need HMAC (and so don't KDF, because the shared secret can be used as the single key).

I'm not a crypto expert, but if your implementation is truly the same as ElGamal, well it is being used. For example, Pretty Good Privacy implementations use ElGamal as one of the options.