Since we do not know what to expect, there is no saying that even the most complex of plans will be fool proof. The slavers will not act according to your plan.
That said, your plan is a good base, however, why would we need to claim we took anyone from anyone? We can simply be representatives of Brethor's group. I also think it would be wise if we have the two elven Wardens, Lalin and Faer, pose as slaves along with the other elves. This way they are able to get in there with us without question and from there can help the other slaves escape.
Since that apparently leaves, you, human Warden, and Hissra and the Karashok as the remainders, we will pose as your Tal'Vashoth body guards. But you must sound knowledgeable when you speak, as the whole plan rides upon you being able to convince the slavers of this.