What Apollo needs to break the status quo is a senate. Surely there's some emergency law that allows a regent to confirm elected representatives, but first there needs to be someone to elect those representatives - annex some of the protectorates directly into the Moon Kingdom, have them elect senators, and have those senators either confirm Usagi as queen or elect Apollo king, at which point he can appoint Usagi as his heir and abdicate whenever she's mature enough to take the throne. The annexed protectorates can then secede and return to the old arrangements whenever they feel like it.