As a disclaimer, although I've been keeping up with this thread for a couple of days, I haven't attempted to go through it starting at post 1, so if this has been said before, I apologize in advance.
Although it would be pretty cool for there to be Legends based on Norse mythology in a Pokemon game, there really isn't any supporting evidence that these mascots are based on the nameless eagle and the four hart that live on top of Yggdrasil.
Many people, around the web, have been saying that Yveltal's eyes make it appear blind, but I've personally found no supporting evidence in either the Prose or Poetic Edda (where we get our information on Norse mythology) that the eagle is blind, it does however have a hawk that sits between it's eyes.
As far as Xerneas goes, there are four harts (male deer, specifically red deer) that live on Yggdrasil and feed on leaves from its highest bows. But it seems that many, again not specifically here to my knowledge, have tried to link Xerneas, who is very much in the shape of a deer, possibly a stag, to these four stating that these harts have 4 different colored gems or stones amongst them and that Xerneas' antler's have 4 different colors. Again I've not been able to find anything in the either Edda about the four harts being special other than their location and food.
Now to cover how it would be possible for Xerneas to be female, which although is different from how mascot legends have been handled before, being genderless. But Xerneas could be base on the Ceryneian Hind, which although admittedly was golden, was both female (a hind is a female deer, used often in reference to a red or sika deer) and had antlers, which was not something that the Greeks were accustomed to. The Hind was possible based on reindeer (the females have antlers all year long unlike the males that lose theirs outside of mating times). The capture of Ceryneian Hind was one of Hercules 12 labors. The myth some times involves Hercules capturing the Hind after chasing it from Greece to the land of the Hyperborea (a land located north of Thrace, home of Boreas, god of the North wind, today many believe that this is a reference to an area near or in the Arctic Circle.
The similar sounding name, the high chance that the games are related to their matching chromosomes, and the fact that (I'll admit that I may have missed something in my research) there seems to be no actually supporting evidence that either legend is related to a creature in Norse mythology, leads me to believe that Xerneas is more likely based on the Ceryneian Hind than any of the harts on Yggdrasil. I've still been unable to find a possible base for Yveltal, as its or possibly his name has not given me any possible connections to any creatures from mythology.