However, we already know one situation that can cause a unit to have it's upkeep be paid by someone other than it's overlord:
being captured. I hypothesize that there are other conditions for that to happen as well, surrender/turning traitor probably being one of them. Presumably, it is based on who is currently in control of a unit. It's not all that unrealistic of a situation. In the real world, the captor must pay for the food of the prisoner.
Similarly, if Parson & co. give GK up to the coalition/Jillian, I would presume that the coalition (i.e. either Jetstone or Jillian) would get both the income and expenses of GK, including the upkeep of Parson, Wanda, and the rest. The new overlord of GK (King Jetstone or Jillian) would need to decide whether to maintain those upkeeps or not.
I'm guessing that for plot continuity (presuming that Stanley does lose GK, which is far from certain) Jillian takes control, spares Wanda for obvious reasons (she likes it, after all) and keeps Parson on to run her armies (kind of the reverse of the relationship between King Jetstone and Ansom).
I also can't see her being any less merciful to Sizemore and any other plaids about who were forced to serve under Stanley; after all, they just did what they had to to survive, and other than Stanley and maybe Wanda, I don't think that any of them would be guilty of war crimes.