Small detail in favor of the frog-from-each-planet idea that I didn't see you mention (sorry if I missed it) is that the mystery frog's eyes flash green-red-blue-purple, ie the kids' colors

I don’t know if it was a “common” theory, but one line of speculation regarding that used to be that the frog was somehow connected to the house juju with the four kids trapped inside (like a kind of anti-English). I guess we shouldn’t dismiss the connection outright, since the house and the frog both seem to be keys to escaping the canon? but you’re certainly right that the eye colours emphasise the final frog as a culmination of all four heroes’ contributions..