I'm not sure the possession is even needed, the Synthese is present the whole time, just in a form that appears altered to Esmenet.

Not that it really matters in the end.

Is a small Synthese strong enough to do this

He slapped her—hard. Esmenet’s head snapped back and to the side. She fell face-first from her bed.

? If not, the glamour would need to be strong enough to completely conceal the use of sorcery.

Also, at the end of Chapter 11:

They (The Men of the Tusk) would joke that Skaiyelt’s wife had to be more mannish than Chepheramunni

Heh. These books are really meant to be read at least twice.

Now it suddenly struck him (Xerius) that he’d never looked into any of their eyes—not once. Matching the Emperor’s gaze was forbidden to those without Imperial Blood. The thought horrified him. Save for Skeaos, I know none of these people.

I now remember noticing this bit on my first read, thinking that this must be significant, and then forgetting it. But on rereading, the revelation that Xerius has been isolated from his entire court besides "Skeaos" is beatifully ominous.