I have not seem hithau prior- did i miss this somewhere or is it used for specific items such as comparing?
Hithau is an emphatic form of Hi, it's not very commonly used. Hi would be used most often in speech for this sort of sentence.
It's included as an alternative here.
Is "fwy enwog" categorically disallowed?
enwocach is the usual form. The other form would be understood, though.
Not sure that it's the more usual form. It's certainly common these days, what with it being a not too short/not too long adjective, and the inflected forms becoming less popular.