> þó að oss veitti það að vilja að vér dræpum Njál eða sonu hans
> even though that happened (veita, Z9?) to us to want that we

(should) kill Njál or his sons

"even if it were to be granted us (even if it should happen for us)
according to our desire, (namely) that we kill Njáll or his sons [...]"

`vilja' = dat. sg. of `vili' "will, joy, desire"

> fyrir margs manns (why genitive?) kné ganga verða
> we will be-forced (see under verða + inf, Z7) to walk on (bended)

knee before many a man

Because they're the knees of the "many men" who they'll be obliged to
abase themselves before, rather than their own knees. The person or
persons grovelled before can be genitive or dative [
http://www.ordanet.is/demo.pl?id=16379 ]. At the risk of giving away
the heavily foreboded outcome, here are some more examples, the first
two from Njála:

(1) Mun nú það sannast sem eg sagði yður að vér mundum mörgum fyrir
kné ganga verða áður en lokið er þessum málum.
"It will now prove true as I told you that we'd be grovelling at the
knees of many before this matter is over."

(2) Guðmundur bað þá ekki ganga fyrir kné fleirum höfðingjum. Kvað
hann það vera lítilmannlegt "og munum vér nú á hætta með þetta lið sem
nú höfum vér.
"Guðmundur bade them not to grovel at the feet of any more chieftains.
He said that was pathetic, "and let's take a chance with the support
that we've got now."

(3) útan þú farir áðr lúðrandi fyrir kné ábóta
"unless you go first grovelling before the knees of the Abbot"

(4) Þegar Símon Pétur sá þetta, féll hann fyrir kné Jesú og sagði: Far
þú frá mér, herra, því að ég er syndugur maður.
"When Simon Peter saw that, he fell at Jesus' knees (we'd say: fell at
his feet) and said: Go from me, Lord, for I am a sinful man."

> ráða undir sig trúnað og fjölmenni
> he thought thus to arrange-for-himself (?) his trust and many-people.

`ráða undir sig' "secure for himself" (cf. `ráða e-t undir e-n' "to
put someone in charge of"; `ráða e-t/e-u undan e-m' "to deprive
someone of something").

`trúnaðr' "allegiance, trustworthy/reliable support" [
http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0642.png ].

`fjölmenni' "multitude, many people", i.e. many supporters, strong
backing.

> alla daga

Maybe MM & HP meant that they talked all day with the implication that
this happened "every day" or however many days they were together.

> Hverja ráðagerð hafið þér nú fyrir yður bræður og Kári?

Notice that bræður and Kári are nominative, in apposition to the 2nd
pers. pl. pronoun `þér', rather than being in apposition to the
accusative `yður': "What plan do you have for yourselves, you brothers
and Kári?" (As opposed to: "What plan do you have, Skarphéðinn, for
your brothers and for Kári.")

> Lítt rekjum vér drauma til flestra hluta.

`til' "in respect of" (Z 4 and 5). A typically sarcastic answer from
Skarphéðinn: (You tell me, oh wise and prophetic one!), "*We* don't
trace/unravel the meaning of dreams (to decide what to do) for every
little thing."