Even though Awakening (and all of the FEs) stories are meant to be taken at face value, it's still fun trying to dissect them. I like discussions like these : D
Anyway. I'd imagine the Avatar's mother is where they inherit their looks from, since they don't look ANYTHING like Validar (Thank goodness).
That being said, we have no idea what goes on with this super breeding program of the Grimleal's, well, maybe we DO but we'd rather leave it unsaid. But it did take a long time to create a vessel worthy. As long as the Avatar's blood doesn't fall into the Grimleal's hands, Grima won't ressurect. Naga saying Grima may come back is her being realistic, because it's definitely a possibility. They can take all the countermeasures they wan't, but maybe 2000 years down the line Grima gets brought back.