Nope. I really don't.
Kill someone far away from civilization and they're dead, dead, dead unless someone can recover the bodies or use a 9th lvl spell. In my game, the PCs are established as some of the most powerful people on the continent in terms of combat (not political) power. That means that the number of people who can perform miracles, wishes and true resurrections are few and far between. There is one other person in Velendo's religion who might be able to true resurrect him.
More importantly, they have made a lot of political enemies, who wouldn't want to bring them back. Sure, their King would if he could, but Velendo is the only person he knows of in the entire country of Gaunt who can cast 9th lvl divine spells. Their surviving friends would certainly try to track someone down who could recover them, but it wouldn't necessarily be fast.
Most of their other allies (such as Ioun) are more allies of convenience or circumstance than best buddies. I get the feeling that the group doesn't trust Ioun (the most powerful wizard on the planet, as far as they know) any farther than they can toss him. Their dealing with each other are very... polite.
(As for Dylrath and his magical scrying mirror, it doesn't find dead things and it doesn't work underground. No luck there.)
So sure, if all of the PCs were killed in the underdark (where they are now) and assuming that their souls weren't trapped or their bodies turned into undead or something similar, surface allies may be able to bring one of them back, who would be able to bring the rest of them back. But it would take a while, I think, and that's fun in itself. When you have a bad guy's plot that moves forward regardless of PC actions, losing a few weeks or months makes a big difference to the campaign!