Well, the other reason why it almost certainly wasn't Serini is because she was a rogue--she couldn't scry on anything to save her life. In addition, if that WASN'T Zz'dtri, then how did Nale know in strip #801 that the Order were in the middle of the desert the week before? It makes a lot more sense to assume it was Zz'dtri scrying on them than to try and twist the interpretation any other way.
Not to mention that the color of the dwenomer used in the scry is an EXACT match for Zz'dtri's
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