Look dude, fact is, we had no idea what was in Firmament, other than a prophecy that Durkon would bring death and destruction when he returned, a letter telling him that he was cleared to return, and a other prophecy saying he would return posthumously. Three separate references to him returning.

Xykon's fortress has Thor telling Durkon to act surprised if it comes up. Nothing about going there, and no information about where it even is (again, featureless plain).

Firmament had to be gone to because of the death and destruction prophecy, or it would be something without resolution. Conversely, Xykon's fortress could never be visited without any issue because we have already had resolution. Nothing is actually there.

You keep insisting Firmament is an analog. It is not.

Is it theoretically possible for the Order to go to Xykon's fortress? Sure. But it is not a loose end. That end was tied up as soon as it wss introduced. It is as much a loose end as the Shojo'a fate. It's theoretically possible Shojo does get revived, but if he doesn't, that's fine, because as of right now it's not a loose end. There's no hanging chad with the astral fortress. It's nearly tied up already. If the author chooses to revisit it, he certainly can make that choice, but it would be a new thread. Not an existing one.