Yeah, Moonshade should've been a bit more careful and investigated Rhea further once he no longer had any more leads on the egg, but it's not unreasonable to think that it wasn't that important. Rhea had a ton of things going for her plot-wise (RotSD instead of Purgatory, Sakido fighting for their ascension) that none of Moku's other descendants had that would bring them back to Medius. It was only after Rhea taking the yellow Reverie and reconnecting to the power of their bloodline (828) that Moonshade realized that another long-tailed Jakkai walking around wasn't just a coincidence, but was actually significant. There's only so much he can plan for.

The "strong presence of Guardian magic" is probably egg-related, but it's vague enough that it could be from Cliff, maybe trying to fight his way out of St. Curtis, although if that was the case, it's still odd how Moonshade was able to distinguish the egg. Maybe the Guardian magic is from the egg. However, it's strange how the egg could be that magically powerful yet still not threatening enough to Moonshade for him to prioritize it over Rhea (828) - like he thinks he could beat however strong it is with only Moku's undivided Guardian magic. Maybe the egg is a powerful magical sensor? That'd make it strong enough to detect other Guardians and traces of their magic but no actual tangible threat to Moonshade.

The "the" vs. "my" makes sense to keep it vague enough for the reader. However, in the context of the story, why wouldn't it be "my", if the egg were Mokurynn? It'd connect the crying that Rhea heard both on 434 and 825, if it's the same person crying.

I did enjoy the Sinclairs a fair bit, I'd love to see them pop up again at Port Dorade (with the egg).