Regardless of who Shiva is, I don't think her relationship was with Nidhogg. I expect it will be with Hraesvelgr. If anything, I am guessing that Nidhogg would have been among those trying to tear them apart.

Im almost certain we will find that Nidhogg's hate goes back further, probably to the Allag. There is no way Dravania was spared from Allag conquest when they were so close to the center of Allag power and that would have almost certainly shaped the views of many dragons.