> Þórður ... kvaðst hafa skyldu rétt af þjófum þeim ...

> Thord ... stated for himself have duty precisely of the
> thieves ...

> Thord ... said of himself to be obliged to duty by those
> thieves ...

> Þórðr ... declared-of-himself (that he/they) should (is
> this seems an example of use of the past infinitive, see
> Gordon 171?) have justice (right, réttr) from those
> thieves, ...

It certainly looks like a past infinitive to me. (I'd make
it simply 'Þ. ... said that he would have justice ...'.)

> ... en smala skyldi reka fyrir innan fjörðu.

> ... and (the) sheep (they) should drive inside of (the)
> inlet. (Z. fyrir innab., prep. with acc. Inside of,
> within)

> ... but sheep should be driven inside (the) fjord.

> ... but (and) (one) should drive (the) sheep (note:
> impersonal construction) inside-of (?) (the) fjord.

<Fyrir innan fjörðu> is 'to the inside of the fjords',
meaning 'inland of the fjords'. The idea is that the
inanimate goods and the people were going by sea, but the
animals would go by land, the long way round: they 'were to
be driven around the heads (= innermost ends) of the
fjords'.

> Síðan stefndi hann þeim Kotkeli og Grímu og sonum þeirra
> um þjófnað og fjölkynngi og lét varða skóggang.

> Then he cited them, Kotkel and Grim and their sons, for
> thievery and witchcraft and caused to defend outlawry
> (caused them to defend themselves against charges of
> outlawry?).

> Afterwards he summoned them, Kotkell and Grim and their
> sons regarding theft and magic and had pronounced
> punishment of outlawry.

> After-that he summoned them, Kotkell and Gríma and their
> sons concerning (the) theft and caused to
> entail-a-penalty-of (varða e-m e-t, Z5) outlawry
> (forest-walking) (on them).

I think that <lét varða skóggang> is 'and asked for a
penalty of outlawry'; see the last example at Z5.

> Þeir bræður urðu óðir við þetta og kváðu menn ekki hafa
> fyrr gengið í berhögg við þau um svo mikinn fjandskap.

> The brothers became furious with this and said men hadn't
> previously entered into an open fight with them concerning
> so much hostility. (Z. berhögg, n. in the phrase, ganga í
> (or á) berhögg við e-n, to enter into open fight with)

> Those brothers became furious at that and said men have
> not previously entered into open fighting with them
> regarding so much enmity.

> Those brothers became mad (furious) at this and declared
> people not to have gone into an open-fight with them
> before through such great hostility (typo: fjándskapr?).

You mean the missing accent? That's the modern Icelandic
spelling. I think that <um> here means something closer to
'with, by means of'; Fritzner notes this sense, though I
don't immediately see it in either CV or Z.

> Því næst laust á hríð mikilli.

> Thereupon (they) were caught by a violent snow-storm. (Z.
> hríð (pl. -ir), f. (1) storm, esp. snowstorm; h. mikla
> gerði at þeim, laust á fyrir þeim h. mikilli, they were
> caught by a violent snow-storm)

> Next struck a violent storm.

> Next to that (in the next instant?), a great storm
> came-on-suddenly (see ljósta, Z4, impers).

Simply 'after that, thereafter', I think; my impression is
that <því næst> indicates the next item in a series and is
more indicative of some relationship between two events than
of a very short time interval between them. Here, of
course, the relationship is causal.

Brian