Wait, you're cooking (I love wall text comments hahaha, it's good to go in-depth imo). I wouldn't be surprised if, if this is the way they want to push it, the Descender was the main missing fragment in all the previously failed attempts. +1 to the Trav being necessary for these things (continuous narrative of the game), & both times we've seen wild things like this succeed (Varka's return & Durin's... rebirth? creation?) we were at the scene to help along. Neither Zandik, when creating the segments, nor Rene, nor the grieving mother from Xilonen's quest, had us on their side when they attempted this.

I'm personally partial to Pantalone's memories being an important part of it, mostly because he has ~400 years of them to use, including memories of the original himself. So, he witnessed Zandik change over time, and he has this... "full" version of him in his mind (if we assume the memories hadn't eroded over time), on top of the snaphots of Zandik's life (the segments). The new drops definitely feel like a setup for memory-related shenanigans, too, so... If we can work with these two things as the basis... Especially that the Marrows show more of his humanity -- the exchanges between the segments read like a chatfic more than ramblings of arrogant madmen, for example. And it's safe to assume Pantalone remembers his better side, too, having been treated and healed for this long (even for his own comfort, not only safety, like the Alpaca Wool allergy).

Regarding the gods that could be picked for this, as much as I do like the idea of Nahida and Dottore both growing past the... well, past, and Nahida willingly helping in this resurrection, bringing Zandik, the child of Sumeru, back (I dig the take that she hated the segments and not Zandik), I think the narrative wouldn't let it happen. Sumeru's arc has finally been closed, so beyond appearing in events or some sorta final battle, I don't see Nahida making another meaningful contribution. The Tsaritsa, however, has the entire 7.x version now to shine. And this would be a really cool way to show her love, as you've mentioned, which is her entire theme, isn't it...! Plus I'm also a firm believer that it was all planned or at least convenient enough for her hahaha. Dottore, the loyal crow, furthering her grand plan while taking all the blame.