> "-doll" on the other hand doesn't seem to have any connection with
> 'light' in any form that I can find. It's possible that it's a
> noun-form with some sort of vowel mutation - but I can't find
> anything that it might come from.

You're right about the vowel mutation; 'döll' is derived
from 'dallr' by u-mutation. Or at least that's the theory
but we have no definite evidence for the 'dall-' root having
the meaning of 'light' in Old Norse.

*dall-ó > *dallu > *döllu > döll
*dall-az > *dallaR > *dallR > dallr

Kveðja,
Haukur