I scaled her head up  to 107%, which would bring it closer to 7.5 head lengths.  I think it looks much more natural ... what do you think?

Looks fine. Scaling up a head kind of reduces the height of a figure in a way. Especially if you lengthen the neck. I struggle all the time with Daz mesh.

I think they went for a monumental/heroic  proportion with their head sizes. I know women who are over 6 feet tall and their head size is the same as one at 5 feet 4".