I think Nwm should have tried to cast a spontaneous version of the Pale, and thereby seal the breach.
I don't know what the DC is for the Pale, but even if (before mitigating factors) it was a hefty DC 400 or so to establish a single section, the DC before the x5 modifier for permanency would be only 80. Backlash reduces this by 42, and 3800 XP brings it to zero. If a suitable symbolic magic item were available (Daern's Instant Fortress? A magic shield?), he might not have had to spend any XP. A 20 hour respite in the siege would have been very significant.
Alternatively, an epic version of Reality Maelstrom could have cleared the battle field, and kept it clear. The Orb of Storms would be an appropriate component for such a spell.
Also, I think that Mostin should have asked the Horror more questions about payment. How are the faces to be transferred? Do they have to be from living people? Etc. It is too bad he never thought about the question of payment when he was super intelligent, and had access to the Web of Motes. He might have determined that (due to their skewed values) a stale prune danish would have been satisfactory payment. I think it can be assumed that multiple human sacrifices (or the personal sacrifice of Mostin himself) is not be the price, as possible futures involving the Horror would have included the life threads of the sacrificed ending in the Horror.
And Mostin definitely should write more scrolls.