Here's how I see it:
Maxillar may be the rightful ruler in HIS mind, but unfortunately, not all of Taldor sees it that way: some see Eutropia's status as a blood relative, or even the fact that she's been heir for longer, as taking precedence over simple physical age. And frustratingly many people just LIKE Eutropia more. It shouldn't matter, but when "which of these two heirs are the legitimate one" is an open question, it IS important. Both have their supporters, and neither can just claim the throne unchallenged as things stand.
Maxillar probably hoped that rescuing Stavian would lend legitimacy to his claims to the throne: both by triumphantly saving the Grand Prince, and through Stavian's direct and unquestioned approval, which he didn't get a chance to publicly receive before the events of the Gala. That said, he didn't want to be known as the person who FAILED to save the Grand Prince, so he kept it secret until he know if Stavian's recovery would be successful.
Unfortunately for him, Stavian came back physically intact, but even more mentally broken than before. Revealing him might hurt Maxillar's legitimacy: his enemies might ask if Stavian was mad even before signing the adoption papers. But having him killed is also a big risk: what if someone CAN bring him back without a body, or even just contact his spirit somehow, and find what Maxillar did? The safest course, unquestionably in Maxillar's mind, is to keep the status quo: keep Stavian hidden and under wraps until he secures the throne and THEN resolve this mess through whatever means seems most expedient at the time.
Meanwhile, yeah, the Ulfen guard searched high and low for the body. But it's not really unreasonable that one couldn't be found. Anyone who wants to kill a noble knows they need to take the body, or else it's just one raise dead away from being an unsuccessful assassination. True Resurrection, on the other hand, is a lot harder to come by as a 9th level spell. And securing a room safeguarded against divinations is probably simple for a man of Pythareus's means.
You COULD just say "No, Stavian actually died": the twist of his being alive isn't really necessary for the campaign (and honestly, my PCs predicted it basically the instant his body was unable to be found.) That said, it IS an interesting character moment to see, not as a surprise, but as a choice: what do the PCs do with Stavian? It's a very complicated situation with no clear-cut "correct" answer and it's the things like that tell us who the PCs really are.
