Heil Alfta!

> How closely (or not close) does Zoëga conform to
> the Íslensk fornrit editions in general?

Very closely. The only (relatively) major discrepancy
I remember is the middle voice ('st' rather than 'sk').

Note that the ÍF editions do not all have the same
spelling - works from different time periods have
(slightly) different flavours. It is the ÍF
Heimskringla edition that I have taken as model.

Zoega outlines some of his principles in the preface:

http://www.northvegr.org/zoega/000_04.html

Kveðja,
Haukur