> en hann bað þá vel fara og varlega fyrir þeim bræðrum.

I think this is Zoega's 12th definition for 'fyrir' + dat. "against"
in the sense of being on one's guard AGAINST, be protected AGAINST, or
FROM, or "for" in the sense watch out FOR.

gæt þín vel fyrir konungi "be on your guard against the king"

beiða griða Baldri fyrir alls konar háska "ask for immunity for Baldr
FROM every kind of danger"

og far varlega fyrir Einari flugu "and go carefully / be on your guard
against Einarr fluga"

But maybe it could equally be counted as Zoega 11 "for, because
of"--go carefully because of...?