RE the einherjar and soulfire, and:
... another WoJ where he said Odin got the soulfire when the christian god took over or something like that...
And then the quote
“Einherjar. Give them a little sip of renewed mortality, and four thousand years of discipline go right out the window”
There are IMO several (relatively-easy) resolutions.
First and most-trivial: Sigrun Gard is likely an unreliable narrator, just like Harry himself. Four thousand years, two thousand years, meh... no difference, really.
Second: while "Christianity" is explicitly 2000 years old, the "Christian God" is explicitly the same as the Jewish God, and thus thousands of years older (the Exodus from Egypt being about 1500BC).
Third: despite what many think, the Norse "old ways" aren't all
that old. The Eddas were 13th C; the "Viking Age" began about year 800; and the oldest examples of Eldest Futhark runes and other "Proto-Norse" inscriptions are from the 2nd C. Obviously, these represented an extant culture & tradition, not something brand-new. But it puts the origins of Norse ways
REMARKABLY close in time to the beginning of Christianity, hmmm?
No reason at all that Young Odin couldn't have made a deal with post-Christ Yahweh, and then Soulfire'd some centuries-old warrior-ghosts (think Sir Stuart) to jumpstart his corps of fighters.