I thought the 'glamour' explanation was used on dragons making themselves look like humans.
Like the inquisitor in the main story line.
That is why they throw people of the Witchdrop, to force them to shed their glamours (.... or die a horrible death. I hope that dinner with Halone is worth it).

As for people turning into dragons like in Stone Vigil HM and Snowcloak, I'm not sure. I think I'll be sticking with Sholah's theory.