The reply to your divination
"Bear the least of Acererak's form, and you can pass through the face of the fiend to the City that Waits. There you must find passage to the Fortress of Conclusion, where you must face the true Acererak!"
Grunther is ready; also True Seeing. Let's go!
OK, Grunther has bought a +4 Circlet of Intellect and enjoys his new (relative) genius. (His Int is now 10.) He has also had his armor enchanted and bought the usual buffs (Cloak of Resistance, etc.).
Unless I see something to the contrary, I don't see why quintessence would interfere with True Seeing.
So: You've cast your divination; characters are ready to go. What next? Let's go! We have an adventure to play!
More about quintessence and True Seeing, also Grunther
Well, the time-dilation effect of quintessence is the sort of thing that TS is designed to see through. In general, TS is a powerful spell; unless there is some sort of specific indication of something blocking TS, I would tend to say quintessence wouldn't block it.
Also: His name is Grunther, not Gunther.
The meeting of the groups
I'm thinking something along these lines. Mato, Solenal (the Radiant Servant, correct?) cast some divinations that warned of the rise of a horrible undead being named Acererak. (Pelor HATES undead, of course.) You did some research and found out that he was a half-fiend wizard from centuries ago, and that he had a famous Tomb in this area. You were told to look for allies; these allies had defeated the Tomb but not Acererak himself, and would need help for the next stage. You made your way to the rumored location of the Tomb, and here you are.
Oh yes: You cast the divination, as specified previously, concerning the City That Waits and so forth.