Hm. I suppose that's what's intended by this statement, "The other conspirators don’t know anything other than what the assassination plot required them to do (including the elf corpse trick, and that Lee Sidoneth orchestrated the whole event). They don’t know about the Ragesians in the Fire Tomb at all." I will say, though, that IMO that is worded in a way that indicated to me the opposite of what you say. I.e., that the conspirators DON'T know anything....including. After all, why is knowing that Lee Sidoneth orchestrated the whole thing something that falls into the category of "what the assassination plot required them to do." But you're right that, without reading it the way you suggest, there's no way to connect Sidoneth.
Isn't Lee also missing at the play? And as lockyreid pointed out, they met the 2 Half-Orcs at Lees House anyway. Lee is also instantly missing after it happened. In The end I guess it does not matter that much if they are out to find him or not, since it is Simeon that points them to the prison anyway. My PCs also did not think at first that Lee was connected, since Giorgio fled and the assassin as well. But they knew the 2 Half-Orcs and had their suspicions. Interesting for me was, that they actually tried to get to the Biomancer because they thought, he played a major part in it all after seeing them all together. Just let it happen and get them summoned by Simeon after some searching to be directed to the prison. Worked well for me I would say.
I ran two groups through this adventure. I didn't emphasize the orcs in Lee's house to avoid obvious foreshadowing, and assassins on the ship were either killed or escaped, so both groups just knew that Lee disappeared. All NPCs were saying that it is Lee's jobs to control the storm, but he's gone for some reason.
I think this made it more interesting, as the parties didn't know if Lee was kidnapped or is actually involved.