He's got several theories and is keeping firm track of the 'none of the above' category, of course.
His top theory is that Opalyn was powerful enough to cut down a Prince despite being greatly unskilled in wizardry, meaning that she's more powerful than a Prince, meaning that the Dread Emperor was indeed from outside Eldrida or maybe prior to its existence and that Opalyn possesses power on the same level as he. She went back to the Facility because she didn't have anywhere in particular else she knew to go, didn't know how to teleport, and wanted to try messing around in lewd dramatic prison after having led a much less exciting life back in her magicless home universe.
His second theory is that people from outside this universe are also cursed by very weird luck, such that Opalyn managed to wildly disrupt lightline charging despite not actually having any more power than a Duchess, causing the Dread Emperor to show up and kill a lot of people but not actually kill Opalyn and send her back to the Farm, or something like that.
His third theory is that there is no Dread Emperor, never has been, and whoever really rules Eldrida was attracted by the presence of an Outsider and then events ensued which got the Farm Prince killed and Opalyn sent back to the Facility.
None of these theories except the first one super hold together.