> Eg mun ríða drottinsdaginn og svo nóttina með.
I think `svo...með' here = "too" or "as well": "I will ride during the
Lord's Day (Sunday) and the night too."

> Skuluð þér þá þar allir komnir er eiðsvarar eruð við þetta mál.
"You should then all have come/arrived [all you] who are oath-swearers
in this case/matter." (Past participle, with the auxiliary `vera'
omitted.)

> þá skal engu fyrir týna nema lífinu
Lit. "then shall nothing be lost except the life" – i.e. "then we
won't inflict any penalty less than taking that man's life". See CV
under `fyrir' "instead". There's another instance of the expression,
also from Njála, quoted there: `þér skulut öngu fyrir týna nema
lífinu', for which is offered the translation "you shall lose nothing
less than your head".