A changeling who assimilated a Borg cube, like Lore with that weekend retreat of Borg drones that one time. Fascinating. That really would explain everything. Because you're right Axaday, the Borg shouldn't need a hardcopy of Locutus. If they really wanted one they could just grow one, and out of all of the DNA in Picard's body, the least useful to them would be the part they wrote in themselves. I could forgive them for not keeping his genome on file, but their own damn code? Nonsense. But if the cube was disconnected from the collective to keep the changelings' doings from spreading, they might not have access to that information. One cube, of course, is all we've actually seen so far.
Meanwhile, these new changelings with simulated human physiology could be the ideal trojan horse - we can't know exactly what would happen if the Borg tried to assimilate one, but it's exactly as likely as any other possibility that the changelings would assimilate the Borg back instead. And who better to push the envelope of Borg biological manipulations than someone who consciously reconstructs their own entire human biology every time they wake up in the morning?
She did sound like Vadic, but I'm still thinking of that line line BBEG said to Vadic about how "your people" are "malleable" and made to bend to power. What do you mean, "you people"? And if it is a changeling, they're going to have a different attitude than Vadic did. Vadic's entire complex is the product of torture, but BBEG uses torture against her? I don't think Vadic would torture someone she didn't consider a deserving enemy.