> veik hvergi af
> ie did not deviate at all (?)

Yes, I think that's right. MM & HP: "word for word".

> en Hallgerður festi sig sjálf ef lögmanni þykir það rétt.
> but Hallgerð if that seems right to (the) lawman (lawyer)

MM & HP: "But H. herself declare her own betrothal - if the
Law-Speaker considers that correct procedure."