The book is wrong. How could Ysgramor be the one who brought the worship when 1. He wasn't the first atmoran to come to Skyrim, and 2. The dragon priests recieved their power from the dragons who were in Skyrim, not atmora. and 3. Because the atmorans killed the LAST OF THE DRAGON PRIESTS when they came to Skyrim as is evident in the seige on the dragon cult?
That also supports that men came from Skyrim, because if they didnt, then how did they have the dragon cult? The war happened in Skyrim, and dragons gave the mortals power and they became dragon priests, which is how the cult started. But if the religion started in atmora, how could they have dragon priests? The dragons were in skyrim, and the war happened there, not in atmora. The elder scroll shows us that. So the dragon cult had to have started in skyrim. Then the nords went to atmora, then they went back to skyrim which is when they defeated the elves. That also explains why the atmorans had the dragon language and the thu'um. And the cult must've stayed in Skyrim, because seige on the cult shows that they were still in skyrim, and they "survived" there, which would suggest the atmorans were not of the cult.
It doesnt make sense for Ysgramor to be in the dragon cult because they killed the dragon cultists in Skyrim, and the religion started in Skyrim, not atmora, since the religion started from the dragons who gave the priests their power, and the dragons were in skyrim.
The journal from the Siege on the Dragon Cult shows that a group of cultists were holed up in Forelhost in 1E 139. That's in King Harald's time, THIRTEEN generations down from Ysgramor. I don't see how that proves that the Dragon Wars took place before Ysgramor's time.
And duh, yeah, we know the Dragon Wars happened in Skyrim. But the Elder Scroll only tells us that it happened there, it doesn't tell us when nor is it proof that it happened before or after some other event, nor does it prove that the cult only existed in Skyrim or that dragons only existed in Skyrim. Your arguments seem to rest on those two things being fact and I don't see how you've proved any of that.