Natural Distrust of the Drow
Certainly, a peace accord COULD work between the elves of Cormanthor, the Dalesmen, and House Jaelre. However, there are lots of things that stand in the way of that accord being accepted and being honored.
1. First is the reputation of the Drow. Yes, House Jaelre is not populated with Drow of the same vileness as House Baenre. However, to all but the most discerning surface dweller Drow are Drow. It is going to take a LOT of persuasion and a lot of time for the Dalesmen to really accept these Drow as different. I use Drizzt as an example. Even in the Ten Towns, where he has been witnessed to perform many acts of heroism benefitting the population, he is AT BEST tolerated for a majority of the time. He can't even freely communicate with The Lady Alustriel of Silverymoon in her own city. Distrust of Drow exists at a deeper level than distrust of Orcs; certainly no Orc tribe would be able to easily enter into such an accord.
2. Second is the hatred between the Drow and the Cormanthor Elves. These two peoples simply don't get along given their millenia of conflict and betrayal (from both perspectives...). Getting these two peoples to work together will be a Herculean task of diplomacy regardless of what is at stake; the difficulty would DOUBLE if the clerics of either House Jaelre or the Cormanthor Elves are active and/or vocal.
3. Third is the Drow themselves. I am not certain House Jaelre would enter into such an agreement with the intent of honoring it long-term. More likely they would push for some-type of "cease-fire" while both sides were dealing with the threat of the Dragon Cultists. This might be an interesting way of going about brokering a peace. Start with the cease fire and use whatever good will was earned during that to push for a longer accord.
4. Fourth is the Cult of the Dragon. Assuming they are moderately intelligent, they would try to exploit whatever partnership existed between the Dalesmen and the Drow. Either by communicating with them in secret to sow discontent or by planting evidence of betrayal by one group against the other (as an interesting twist have a drow appear to have been betrayed by the Dalesmen...).
However likely (or desirable) long-term success is, there are LOTS of great role-playing opportunities as well as a chance for characters with pumped Diplomacy and Sense Motive skills to get a chance to use them. Most importantly, this story arc looks like a blast to run and play.