Well first off that would be impossible because Maggie has more loyalty to Stanley than to Parson because he's the leader of the side. She wouldn't actively use a spell to lower her loyalty unless she believed it was in Stanley's best intrests and I doubt there is any way she could convince herself that. (Unless she used thinkamancy on herself but then we get into a paradox of sorts.)
Actually, that shouldn't be to hard. Maggie currently believes that Stanley is giving bad/poor quality orders. Parson could probably convince her that giving herself (and other's) the ability to disobey (and withhold info from) Stanley would actually be to Stanley's long term benefit.
Now, that isn't technically dropping loyalty. It is reducing automatic obedience. The modified units would still want to achieve Stanley's goals.
There was a Greg Egan story (spoiler) where people were modified to be loyal to "the company" and they broke the loyalty trap by redefining "the company" as all the people who were modified to be loyal. Since, they were the ones who had the best interests of "the company" at heart, they would the ones who should decide what the best interests of "the company" were.
Maybe a similar trick could be used by Erfworlders, they could say that their loyalty to their side is higher than the loyalty to the mere Ruler of the side and then proceed to something like democratic rule where the ruler had to be loyal to the side itself.