Fuego is Spanish, yes? So "FWAY-go" is right.  If it were Latin, it'd be ignis or flamma, or some variation thereof. (I think fuego is probably actually derived from the Latin focus meaning 'hearth' or 'fireplace', just as luego is derived from locus).

Forzare is NOT Latin; there's no 'z' in Latin--it's only used to represent the Greek 'zeta', but those are just loan-words.  It's probably related to the Italian forza with a Latin -are suffix to make it verbal (why he wants it to be an infinitive, I don't know--but there it is).  Latin for what he wants here is probably something more like ictus or impetus, since the spell seems to imply a physical movement/blow.  Not really sure, as I said, why he went verbal here instead of a noun like he did with the fire spell.

At any rate, I agree--James Marsters' pronunciation of "for-ZAH-ray" is probably correct.