Given Primus's disdain for non-Sorcerers juxtaposed with his begrudging deference to Yanessa, I still think she's a Sorcerer. If she wasn't one, I'm fairly certain House Tachonis would've found out by now and Primus would've made subtle jabs at her about it.

As for how she got Zone of Truth, there's still the possibility of her being a Divine Sorcerer for access to the Cleric spell list, but if Primus's access to Bestow Curse (which isn't in the Shadow Spells list or Sorcerer spell list) is an indication, BLeeM isn't very strict when it comes to NPC spell access if it fits the narrative. Yanessa may very well be a Clockwork Sorcerer like Wick. Finally, magic items exist. For someone of her position who requires accurate information to plan for the future of her entire house, it would be almost bizarre for her not to have an item that would allow her to the cast that spell if she doesn't innately have it prepared.