I'm in the middle of re-reading the whole story, again
A thought did cross my mind, however...
at the point where Mostin, eager to try his Greater Planar Binding spell, no doubt

encourages the group to summon Rurunoth, why were the party allowed the luxury of time and control over the manner! Mostin even wanders off to gather spells, leaving the others holding the baby. But my GM, I'm sure would never let me get away with this! Rurunoth does not exist in isolation - Grazzt will immediately notice his absence, and knows who Eadric is and his part in Nahael's atonement. The characters haven't bothered with Mind Blanks, or even Nondetections, maybe they decided it wasn't worth it. Point is, wouldn't Grazzt have ordered a rescue attempt? Maybe he doesn't care enough about his leftenants? Maybe he was distracted by arranging for Ainhorr to meet the solars. Very fortunate timing for the party, if so. I get the impression Rurunoth is an important pawn in Grazzt's schemes, so I don't get why he was abandoned?