|Darksol the Painbringer|
Darksol, your talking about phrasing and parsed snippets again. The act of casting the spell takes the place of the off hand weapon in Spell Combat. Period. Whatever you see in the FAQ or elsewhere that seems to change this simply is not there.
Now, if a spell is being held, one can use the effects of that spell (to attack) as one of the attacks used while doing the Whirlwind attacks, but it would not allow the casting of the spell, as that ability (Spell Casting) can not be used with the other (Whirlwind Attack). They both use the same action. (full round attack)
The Magus can, however, use Spell Strike with a held spell during either action.
But they aren't mutually exclusive in terms of action economy.
Spell Combat is a Full Round Action clarified to be a Full Attack Action for effects reliant upon Full Attack Actions per a relevant FAQ.
Whirlwind Attack has the text "When you are making a full attack action," in the very first sentence of the feat description.
Therefore, the issue is not with action economy, because one is the action (Spell Combat), whereas the other modifies that very same action (Whirlwind Attack).
I mean, by that logic, you can't Fight Defensively and Full Attack in the same round, because both take a Full Round Action to do, since according to the rules (and your interpretation of said rules), they're mutually exclusive because both require Full Round Actions to do. Right?
Except, if I know you well enough, you'd say "Actually, they can be used together, because Fighting Defensively only applies modifiers to your attacks and AC," to which point, you're argument is hypocritical and thus fruitless, since that's basically all Whirlwind Attack is doing in relation to Spell Combat.