Selucia was offered her choice of posting on the day Tuon was presented as an adult to the Court of Nine Moons, three years before WH, not when she became Empress. Until then, she had been Tuon's guardian - surrogate mother/teacher/co-conspirator/secret bodyguard - but there's no evidence that the post means she was property. In fact I find it very unlikely - Selucia's job was to raise Tuon and manage her household until she turned sixteen and took over the responsibility herself. That implies a great deal of influence and authority, and requires the ability to stand up to and refuse requests of anyone short of the Empress herself, since the greatest threats to a young Tuon would be her own immediate and extended family.

And on analysis the line 'Selucia's mother had given her as a cradle-gift to Tuon', is puzzling. It almost certainly means that Selucia was born into a wealthy and influential family, likely High Blood. Who else would be in a position to 'give' Tuon anything? But then it says 'the first twenty-five years of Selucia's life were in training for that task', which doesn't make sense - her training would have had to start decades before Tuon was even born. And even if RJ meant 'given to the Empress, to give to one of her children', the timelines still don't mesh - if Radhanan recieved infant Selucia when she was twenty years old, that meant she was forty-five before her second daughter was born, and had many more children after her. Not impossible, but very unlikely. It seems more probable that there's a custom of giving children to 'the Throne' to be raised; presumably, every child of the Empress, maybe even the whole extended Imperial family, is raised by a guardian rather than parents, so they'd need to have a good few trained and ready.

So. Yes. I was trying to say that there's no evidence Selucia was property during her time as Tuon's guardian, and in fact I believe her status at that time to have been very high. But she definitely became da'covale on asking for and becoming Tuon's maid. I believe there's a Mat quote about Selucia considering herself 'owned every bit as much as Tuon's horse', and thinking it right and proper. But you're right - I misremembered a quote, and there's nothing to say a Truthspeaker can't be da'covale, only that they can't be coerced or punished. So Selucia is still da'covale, but in a radically different role (and with a great deal less status, once Fortuona finds a new so'jhin - look at how Falendre spoke to Anath)