Because a thousand years later he won't be remembered and he'll just drift into nothingness?
Maybe. It could be an Oblivion thing.
Perhaps Ferrovax has some sort of Duty not in the NN?
Maybe, but I thought the Dragons (like most gods) were basically 'retired'. Even Odin describes himself as "mostly retired" and he seems far more active than most.
As another guess, if the Fomor get their way, one side or the other may annihilate each other entirely so there's nothing to come back to in a few centuries.
I don't see how. Too many of the supernatural beings are dependent on humans in one way or another, so they wouldn't wipe out humanity. And mortals couldn't definitively 'win', as too many beings are either Immortal or can retreat to the Nevernever.
As a third guess, Ferrovax might prefer the Accords to be in place.
Apparently he does. I'm just not really sure why. It seems kind of weird to me... I mean, they're specifically Mab's Accords, which means they must be a new innovation on Ferrovax's timescale (Mab is relatively young as immortals go - she's younger than Nicodemus).
I'd kind of expect an ancient force of nature to resent some new set of rules that constrain him to deal as basically an equal with wizards and vampires and ghouls and other beings that are comparative gnats... I mean he seems pretty disdainful of humanity in GP.
Because he is keeping an eye on Odin, they counter balance each other if he is indeed the Midgard Serpent.
Not really, the Midgard Serpent is Thor's fated foe, not Odin's. (Odin's is Fenrir.)