no intrinsic supernatural powers
A mortal who can take sponsor debt has no intrinsic powers. Intrinsic, by definition, means a power belonging wholly to the mortal, as part of them. Sponsor debt is not something that is not a power of the mortal, but instead an outside source granting them aid (by definition, not intrinsic).
When you take a power, like Cassandra's Tears, you are no longer pure mortal, you have your own supernatural power (even if it isn't useful). Such a character is clearly no longer pure mortal, they have an intrinsic power (though I would likely let them have the bonus, for game balance reasons). On the other hand, entering into a bargain with an entity does not give you powers by itself, you have no powers, you cannot do anything special, you just sometimes get a boost (invoke an aspect or such, which is clearly not a supernatural power) to whatever normal mortal thing you are doing in exchange for your "help" later.
Weather or not you choose to give out the bonus refresh in a given circumstance is your choice. As is what you want to call a pure mortal. But the reason the bonus exists in the rules is clearly for balance reasons (or at least the decision to add it is motivated strongly by balance, among other possible factors). If a character does not have any useful supernatural powers, they are going to be weaker than one who does, that is why they get a bonus in my game (not for flavor reasons).
Also, I agree demonic co-pilot is a horribly worded power and would just be better off as an aspect and sponsor debt. You can invoke the aspect to help you when you are acting in accordance with the co-pilot's agenda and you get compelled when you aren't. If you wanted to make it cost something it would be easy to allow the demon to help you when you act to further its goals, look at a power like Righteousness, which is kind of like demonic co-pilot where the co-pilot is God, just re skin as appropriate (change conviction to some other skill, change out the burst attack for some other ability you can use, maybe after you savagely kill someone).