I've my doubts. Tillix might be only a CR 8 creature, but he excells at one thing, and that is what he's doing. He has too many advantages:
Total ability to dictate the time of conflict. (ie, the PC's will never catch him off guard, the best way to get assassins)
Only two people will be able to see him (Bard and Dru/Wiz) if they have See Invisible cast at that time. If they don't they'll be able to see him otherwise, and warn the fighters about where he is, but they wouldn't have stopped the initial hit.
Here's what I would do if I were Tillix:
While ethereal, get right above a sleeper. Free action to material plane as I'm hitting on a Coup de Grace. Automatic hit, auto crit, and will deal 4d6+12+poison. [The poor sap who gets hit by this will have on average a 26 DC fort save to survive, even before the CON poison lowers his HP and fort save.]
..........Suprise round is over, first normal round........
If I win Initiative, make sure the guy I hit is actually dead. Then go ethereal. If he's still alive, kill him.
If I lose Initiative, go ethereal as soon as possible, and wait.
..........End of Combat............ until I decide otherwise.........
Which is what you had outlined in your first post, but is there any real defense against this? They will have to stay awake all the time, and become fatigued or exhausted, just to protect themselves. The fighters will have to sleep in their armor if they want to be able to fight Tillix.
The rogue, the bard, the cleric and the dru/wiz will fall to the first coup de grace fort save Tillix gives them. The rest can be dealt with from there since they have no methods of seeing where the spider is.
His motivations are to complete the job and get cash. Unless you specify "don't kill them" to Tillix, there is no reason for him to not be incredibly successful in his attack. I would suggest using a different creature who can't phase and immediately become immune to material-plane attacks.
If you use him (because he is really really cool), I would buy him a brooch of shielding: he knows he can be hit with force effects while ethereal, so there is no reason he wouldn't block the most common of them. That, and they're cheap.
. I've added that to the Round-by-Round Tactics section. I think that setting fires to create confusion may become his normal MO. After all, an all-consuming blaze hides a multitude of sins 
