I have yet to finish the first part, but I am curious about the MC lineage. At first I was dragon or fire elemental, which is what's mentioned, but there are other flaming races out there. So I thought... demon? But maybe its devil. Its interesting how both of those species are differentiated. Also a curse on the entire plane? Wow. The other thing of lore that caught my eye was the whole bit to do with the "gods" and the Lady of Graves role whenever one universe dies and the next is born.
I ponder, would the diverse genes in the MC the mystery lineage could override the inbreeding issues. Some animals do not suffer much from these issues, mostly because their huge genepool. Humans, us in the Realscape, do have a very limited genepool to start, the bottleneck a few thousands of years ago made sure that our genetic diversity is non existent really so risking bottlenecking ourselves even further is a bad idea. Only thing adding some diversity are viruses, as recently discovered we inherit 8% of our genes not from humans but from viruses. Makes me wonder if that is by design, intervention maybe, some non-human help? A prank? LOL.
Eitherway, with the further genetic variance in the setting, I do not foresee much issue with inbreeding problems, even with the decline of male birthrates. I think I am overthinking the story logic a bit but whatever, its fun. Also I am thinking magical beings are not entirely beholden to genetics.