Would Viconia be risking discovery (as a follower of Shar) and therefore death if she reveals that the rest of the party are surfacers? Of course. Would she be at any greater risk than as a member of the adventuring party who's been tearing down every obstacle that's been holding an entire Drow city in check since who-knows-when? If activity like that doesn't provoke Lolth's curiosity, I don't know what would. Also, you're assuming that Viconia just stands in the middle of the Ust Natha Tavern and starts yelling that her traveling companions are surfacers. She'd have to be an idiot to do that--what she would really do is tell the news directly to Ardulace or Phaere, preferably after she's learned of the plot that threatens to betray them to their respective death(s). The Matron of House Despana could arrange a deal: Information in return for Viconia's safe passage, and the rest of Ust Natha need know nothing about it.
I understand the whole spiders-muttering-in-the-ear thing, but according to what's in the game, it's possible to lead a troop of all-Good surfacers through the gate, have them wear brightly-colored, sequined jumpsuits that say "LOOK AT ME" as they slay every challenge in the tavern, then become celebrities for wiping out three successive caverns full of enemy critters, engage in hot wild monkey sex with one of the city's more insightful bigwigs, indulge in some casual Lesser Demon Lord summoning, and finally rampage through the whole city, slaying every man, woman, and child--and throughout the whole thing, Lolth doesn't send so much as a piece of junk mail. So I think it's safe to say that, as far as the game is concerned, Lolth isn't aware of anything except followers of Ghaunadar.
Zyraen: "A quest that destroys the long-term usefulness of an NPC is not useful"--you're quite right. But then, if players knew that they would have to use Viconia correctly if they wanted to use her at all, then that would help encourage proper roleplay, and what's wrong with that? One of my all-time least favorite parties was a guy on Ironworks who said that his current team consisted of Korgan, Viconia, Edwin, and a LAWFUL GOOD Kensai-Mage.
Besides, a Viconia Betrayal quest need not go as far as the actual betrayal--even a simple dialogue that said nothing more than "You will not kill any Drow in the challenge rooms, or I shall leave the party," or "You shall not take any action to disrupt the plan to summon the demon, or I shall inform the Matrons of your true race," or "We shall escape from the city as quickly and quietly as possible, or I shall turn against you," would be a huge improvement over the current setup, in which the party casually stomps all over practically everything Viconia holds dear--and not only does she not raise any objections, but she actually helps. At least when you have Keldorn attack any member of the Radiant Heart, he turns Hostile to you.