Sometimes the rules are ambiguous. In play, the best call is probably whichever one keeps the game moving.
On a related note, spellcasting is as powerful as the GM makes it. If you're too permissive, it can get crazy strong.
Orbius, mental evocation attacks, Feeding Dependency, and Demonic Co-Pilot are best avoided.
If one of your players starts creating their own plot-line, run with it. Player-driven stories are the best kind, in my experience.
Don't pay too much attention to the opposition-building guidelines. They're not reliable.