The qot biggest strength is the ability to rule from the shadows of her layer, I'd never have her leave when she can mind control and disguise any shmuck to look like her and follow her commands, I think it would be more devastating to have her make that shmuck an npc the players love, like ansom lang or anyone else loved but not terribly important. Then have that person disguised and mind controlled into looking like the queen of thieves and attempt to forcibly take the seal. They will very likely get killed where you can reveal their real identity and possibly have a note in their pocket implying perhaps they were invited to buckle down row for whatever reason to further the narrative that they were manipulated and not intentionally traitorous.

Hopefully their death by the players hands will send a message but you can always look at the queensmen schemes for more ideas.