So is there a set tradition that non-Ashlander Dunmer use for burial? The urn would lead me to believe they use cremation, but their resistance to fire kind of puts me off of cremation. Is it possible that each family has a different tradition? Any answers would be welcomed.

Fire resistance is a gameplay thing, its not like Dunmer in the "real world" could walk through furnaces unscathed. Afaik, cremation and sacred necromancy (being turned into a bonewalker/skeleton/ghost etc. to serve your family in the afterlife) are the two most common ones.

There are a couple books on it in Morrowind. I've gotta run and do stuff now, but I'll link ya to them if no one does before I return.