If it's a debate, then it's the kind where one side is clearly right.
Uh...check, say, Inhuman vs. Supernatural Toughness. That's clearly a direct addition.
And yes, I know the book says it's equivalent to taking Channelling and Ritual for 2 Refresh each. But that isn't true, because that isn't how Sponsored Magic actually works. The book contradicts itself. It happens.
No, it really doesn't. It says that Channeling and Ritual are each half of it's cost, which is consistent and makes sense, the additional bonuses are paid for by it's nature as...not entirely under the player's control.
The agenda, incidentally, consists of Compels and doesn't affect balance.
Not entirely it doesn't. Check p. 287 once again, you
cannot even cast spells with it that don't align with it's sponsor's agenda, making entire realms of spellcasting off-limits. It's that kind of limitation that pays for it's assorted bonuses. There's also Sponsor Debt...but that's a different thing entirely.