When most people say hardcore, they think of bands like Minor Threat, Bane, Cro-Mags, Bad Brains, Cross Examination. I have never heard of happy hardcore.
Metalcore is obviously a merger of hardcore and metal that is different from crossover thrash. Metalcore can basically consist of two different things. There are bands like Integrity and Converge who choose to use metal riffs with hardcore rhythms and vocals, and there are bands like It Dies Today and other Trustkill/newer Victory bands that do it as a way to have an instant fanbase without having to be different or sincere in any way.
Edit: I didn't realise this thread was older and wasn't sure if the discussion was still going on. Apologize for bumping a bad thread.