That's right. Compare the nouns: fyrirsát (f.) and fyrirsátr (n.),
both meaning ambush.
> Njáll lagði ekki til þess
> Njal lay nothing to this (he did not object)
Yes: "Njal said nothing (on the matter)" (compare: 'leggja fátt til'
"to say little, be reserved"). More generally, 'leggja e-t til' = "to
contribute something" (e.g. advice, help, interference, good words or
bad...).