He could simply have a couple of bodyguards. Possibly his conversation could be triggered to start as soon as somebody crosses the threshold to stop being surrounded.
The team had already given him two spiders - gone now, I suppose. The module made Lareth to be just a punk - and to die like one, alone and away from his men. Perhaps module consistency should give way to good DMing, which is why I relish the concept of the Reactive mod. I think the voiced dialog would get screwy if Lareth could begin in the main room. I suppose the best suggestion came from one of the modder who mentioned a guard going to get Lareth, thus bringing him into the fight. I think Sitra said that would be fairly simple to do. My 2cp - It would be cool if there could be a dialog interrupting combat or a floater saying, "Get the Master!" followed by "Master! Intruders! Help!"
Lareth is a arrogant, butthole jerk and deserves to die(good align), but he is very talented and should be able to put up a good fight. Better ac and a rod of smiting, or anything to make a better fight of it.