Here's a fun thought exercise: if the other characters were debating this scene...

would guess that it's Julia; the tone of the scene to this point requires it to be genuine.
would insist that it's not Julia, seeing another shell game in progress, and insist the most likely culprit as Serini, testing Roy's commitment to "the best solution for the most people", even if it goes against his personal preferences.
would just cast True Seeing, obviously, but if that wasn't prepared then he'd probably insist that people judge the words themselves rather than focus on who is saying them - something that would have helped immensely when he was the one who needed to be sniffed out.
would say it's not worth the risk to even think about it and insist on blocking Roy's mind from the spell.
wouldn't publicly take a side at all, still somewhat paralyzed by prior major mistakes in his own recent past.
wouldn't have to guess because if it is Eugene, one look at the crown would end up dispelling the glamour.

I'm with Elan.

And on the subject of child sacrifice when literally weighed against the fate of multiple worlds - worlds which happen to include billions upon billions of children, including the one to be sacrificed anyway - it's something of a no-brainer. To even consider how I would feel about it is inherently selfish, because my guilt-wracked conscience is not worth countless lives, and short-sighted, because not using every tool at one's disposal is essentially the same as choosing to imperil everyone. I'd rather sacrifice one and look him in the eleven eyes while I do it than coldly sign a trillion anonymous death warrants.