> Og ef þér rekið eigi þessa réttar þá munuð þér engrar skammar
(skömm) reka."
> And if you do not pursue (seek vengeance / redress for) this claim
then you will pursue (seek vengeance / redress for) no dishonour

Yes, see Zoega 'reka' 8 + gen. "pursue, take vengeance for". He cites
what looks like this very quote, in fact, except that his version has
gen. pl. 'engra skamma' "no shames."

> en því var ekki vant.
> but (and) in that (it) was not usual.

I think 'því' = "that", refering to the phenomenon itself. Impersonal
construction with the neuter form of the adjective: one was not
accustomed to that, i.e. that was unusual. ('vanr e-u' "used to
something".)

> Helga brá ekki við.
> Helga moved (reacted) not.

"Helgi didn't react" (Njáll's son Helgi). With the verb 'bregða við',
the person who reacts is dative (see Zoega 9).

> að þá veiði bæri eigi undan (is this an impersonal construction?)

That's right, see Zoega 'bera' II. 2. These examples are a little
below half way down the left hand column of p. 49 in the edition I have:

veiði (acc.) berr í hendr e-m
"game falls to one's lot"

e-t berr undan
"something fails, goes amiss"

LN