Thanks for bringing all those dates to the table, it provides clarity.
if the map case were open, the parchment would have been destroyed from two millenium of deterioration at the hands of the elements.
Not necessarily. It's in a library that was run by sorcerers and is surrounded by enchantments. Also, the seal is broken not the map case and again, it's probably enchanted.
Invoking "magic!" here is pretty legitimate, especially considering Celmomas intrusted the map to
Seswatha. A simple mundane protection as lame as a seal would not be sufficient to someone of his caliber.
Ganrelka is also older than Nau Cayuti, and was born only fourteen years after Celmomas, iirc. also iirc, Nau Cayuti was born when Celmomas was in his forties.
Sorry, I meant to say Ganrelka is the older brother.
I made that same mistake. Older brother

Which means Celmomas would have been 15 when Ganrelka was born and 30 when Nau-Cayuti was born. Having a legitimate child at 15 does seem unlikely, but its possible.
If this was mundane medieval era where the average lifespan wasnt particularly long then I would agree, but its not. We have lots of instances of people living well into their 80's and far beyond. There probably wasn't a huge press to have offspring at such a young age. Like you said, its possible, but I doubt it.
Note that Nau-Cayuti was born of Celmomas' favourite wife, Sharal.
Very important. This points to either polygamous marriages or multiple mistresses, and if the High King has them, then you know others have them as well.
Depending on what makes the Anasurimbor line special, be it blood (literally blood, or genetics) or the Name itself, "an Anasurimbor will return at the end of the world" could point to a vast number of people.
There's also the other Anasurimbor line (Mygella and Nimeric) - but Aorsi fell in 2136. (Mimara could potentially be a scion of that line too.)
Alternatively, Scott just screwed the dates up a bit and it escaped editing.
There are still so many questions, and I would not be surprised if a date got messed up here and there. Its probably difficult to sort through a timeline of 4000+ Earwa years that was developed over 18+ years of Bakker's life.