Hello folks. I've got a tentative group together, and I think I want to run the Council of Thieves AP for them. However the group wants to begin at 3rd or 4th level instead of slogging through the really low-level stuff. Having read the reviews of Bastards of Erebrus, I'm actually inclined to agree. However, is The Sixfold Trial suitable as a starting point, or will I be missing important background without the first book?
I think starting at The Sixfold Trial could be a good choice. Just tell your players they are part of a fledgling resistance group called the "Children of Westcrown", with appropriate introductions with the group as roleplaying encounters.
You might want to work in the last encounter from Bastards of Erebus, as that is the first encounter that suggests a higher power (the Council) is behind what is happening in Westcrown.
Meeting Thesing is the only other relevant background encounter, but it is low-key enough that it probably won't matter if you omit it.
There are a lot of people you meet in the first book. By the time you do all the work to work them into the story, you might as well have just run the first book. It goes by very fast already, and there are only three sequences of encounters that aren't optional side treks. I'd recommend just running it and cutting out a few encounters and hand-waving the leveling.
The first time I ran CoT, I skipped Bastards because the reactions I got from my players were along the lines of "eh...rebellion, WOAH theater!"; three bards, a rogue, and an Asmodean priest (each had very relevant and unique backgrounds). I started them at level four and ran a very low level encounter to tie them all together. Afterwards they heard news of the Trial auditions, went in, got parts, rehearsed, related side-quests, performed (and survived), and got invitations to the Folly. These events required more exposition and roleplay, but they worked out better than any other incarnation of CoT I've run since.
Definitely worth a try. Good luck.