Honestly, I doubt either is outright a spy. (Though I haven't played the latest release yet, so any new events might contain further crumbs I'm not operating with.) X'zaratl's whole schtick being an act would make her "I'm a Rowlexia shipper!" redesign a lot weirder and less interesting. They could maybe write it so these two things aren't mutually exclusive, but her acting as NTR route detox makes that pretty hard to do. (This is never mind the fact that it would have to completely invalidate her functioning AS a detox mechanic, I assume? Since I don't go those routes, my only experience with that is from skipping through the sex scene via the replay mod.)
As diebesgrab notes, Liurial is basically the starter kit for a more maledom Rowan (He has some earlier scenes that brush up against that, but nothing nearly as overt as Liurial). "Character tries to spy on dominant character by pretending to be submissive to them, gets caught and dominated harder" can definitely work (Filiane is my favorite Bunny Black 2 character for a reason), but putting her in that Intro To BDSM role and re-writing her scenes to contain actual aftercare makes it a lot harder to do that in a satisfying way.
However, this all ignores that there's a third option. Either or both could be part of Jezera's plans without being her agent. It's entirely possible Jezera passed on Liurial knowing she'd probably seek Rowan as her next choice instead of going to Andras, at which point Jezera can try to use Liurial (knowing or not) as an in. Or as just another hostage to threaten should Rowan think about betraying the twins. Meanwhile if Jezera wants to corrupt Rowan and Alexia (Spoiler: Yes), X'zaratl's obsession with their relationship and desire to "broaden" it is a useful secondary angle of attack. Even if the couple resists Jezera's "I want you to be my toys" line of corruption, X'zaratl's earnest fangirling corruption could soften them up for her.
There's a lot of grey between "spy" and "not part of Jezera's plans to screw with Rowan/Alexia."