My group used Occipitus as a new base of operations. Using the template for the new ruler, they found that they saved spellcasting by having the Smoking Eye plane shift (perfectly) to where they needed to go (as long as it had been seen before). Especially after the assassination attempt early in Chapter 7, they were glad to have a "safe" place to rest, even if it is a plane of the Abyss.
This does assume that plane shift is available to the group to return to Occipitus. Possibly a magic item could be "bequeathed" to the new ruler to allow him to return. (The new ruler in our group eventually wished for the power to return to the Occipitus so that she was not dependent on the party's cleric. This wish took place on Walpurgis, which is also the day when the tree's ritual would be completed)
Another way to look at it is that once Adimarchus "dies" elsewhere, his body will go back to the Skull of Occipitus. A final fight (or resolution of his madness and soul through visions) is possible.
Occipitus does not play a large part in the rest of the campaign until they start searching for Adimarchus. I played up the tension in Chapter 7-10, making it obvious that time was of the essence. This put pressure on the new ruler to choose between helping their companions or staying on Occipitus. High-level characters should have to choose between their "kingdoms" and their friends' goals.