> "ef þér Sigfússynir skuluð ræna þá."
> "if you sons of Sigfus should steal them

"rob them" (see my comments ). 'þá' "them" = the victims. 'ræna'
takes accusative for the victim and dative for whatever they're robbed of.

> "Það spara eg eigi," segir Gunnar.
> "I do not mind" says Gunnar (or do not refuse)

More like "not refuse": I won't shrink / hold back from that.

> Njáll mælti: "Hér er hey og matur er eg vil gefa þér. Vil eg að þú

leitir aldrei annarra en mín ef þú þarft nokkurs við."

> Njal spoke " Here is hay and food that I want to give you. I

wish/hope that you won't ever ask for help from anyone else than
myself if you (have) need

Yes: "I wish that you never seek [any] others except me if you are in
need of anything."

'leita' and 'þurfa (við)' both take genitive, hence 'annarra' "others"
(i.e. anyone else), and 'nokkurs' "something, anything".

> Njáll var á þingi og synir hans. Þingið er kyrrt.
> Njal was at Thing and his sons . Thing was quiet (surely most uncommon)

Ominously so? þing + the definite article -ið = -it "the thing/assembly"

> vangeymslu
> carelessness/accident (?)

Carelessness, negligence: van- = "lack of"; 'geyma' "to watch, heed,
keep".

LN