Any canon that is not official canon is head canon. Everyone is welcome to their head canon, but you can’t use it in an argument.
Conan stuff in the comics can’t be used by Funcom (at least not directly). Anything original in the comics is not covered under the Conan IP and so is not owned by Funcom.
I disagree that the way spells are cast is bad. They are intended to be slow to some degree because sorcery in the lore is ritualistic.
I agree that they are, in general, not worth casting though, but because most of the spells are underwhelming rather than the casting requirements are too cumbersome.