I find this really interesting, but have no idea how it works.
What happens if Oathbreaker is used on my "oath" to pay the cellphone company a certain amount each month in order to receive service?
Moving up the scale what happens when it's used to cut the obligations of debt between countries? The probable answer is that the world's economy collapses.
This probably gets even worse with far who draw their identity so heavily from the ideas of debt, obligations and oaths.
So I'm with Ophidimancer here, I don't really see it working for a new sponsored magic, unless it can only be used to immediately accomplish the goals of the sponsor's agenda. There would be a really tight limit on credit, and the compel would occur at the same time as the casting.
This changes to regular sponsored magic deals if the Minotaur is simply rearranging where debt lies by cutting former oaths of fealty to other Courts and accepting and giving new ones to his own. Of course this means his court will have a total breakdown in relations with other courts, the Winter/Summer conflict will become a three way, and the Minotaur will either win, or get obliterated by gajillions of hate filled free agents whose bonds of oaths he's cut.
If it changes to regular sponsored magic it will probably look like a weird hodgepodge of Summer and Winter since it could signify the endings and beginnings of things, and of course wildness. Probably less elemental though, and not growth or decay.
Fae technically wield mortal magic already...sort of. Jim has said fae are just a tiny bit mortal. Obviously they aren't mortal enough to have souls, or free will, but that may simply be the result of taking their magic and their nature and binding it so tightly to so many restrictions in order to increase its power. Whether or not this has changed their magic that it has become too completely other, or they are still capable of the full spectrum of magical options is up to you. In fact, an unbound fae seems to me very likely to start right back at the beginning of magic when all the options are still open.