We ran through Session 7 last night, and it was hilarious.
Spoiler:
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I must admit, TheGimper and I sat out most of the combat. Why? Because we betrayed our allies and slipped around the sentries, heading off to find Valan Jaelre on our own. We're both Bregan D'aerthe, and we both felt that getting the artifact was more important. Also, we're both ideologically opposed to the Way of Lolth overall.
We also didn't know the other one was a heretic until after we'd escaped the combat. I think we were both planning to either convert the other one, or kill them in the stairway.
Anyway, DMs may need to plan for this to happen. If you have returning players, you should know by now if some of them are likely to go against the party, or if they're going to stay on the plot bus. If they're on the bus, the encounter should work more or less as written.
If you want to force the party to stay together, don't split them into the two starting boxes, and keep the sentries back until the party closes in to engage them. TheGimper and I would have had a very hard time if the rest of the PCs weren't drawing fire on the other side of the room, and if the sentries had held their ground. As it was, the sentries moved up, because they are melee fighters, and TheGimper and I were able to slip right past them using invisibility/stealth and cloud of darkness.
Note that the spies can start the encounter hidden and use invisibility+move every round if they don't attack that round. So they can stay invisible through the entire encounter, and just report back anything that happens.
If your party is likely to split/betray each other, keep the invisible spies hidden until it looks like the non-betraying PCs are going to win easily. In our fight, with only 3 PCs participating (we held off from doing a lot of PvP, though TheGimper did fire a couple of shots to screw with the other priestess PC... the one who's loyal), everyone was bloodied, but nobody went down.
We agreed that, in terms of ideal tactical placement, the sentries and the minions are in the wrong places. The minions should be back, because they are ranged combatants, and the sentries should be forward, since they are meleeists. I don't know that changing that would be a good idea, however-- sometimes inefficient tactics are used to reduce the difficulty of an encounter that would otherwise be overwhelming.
