> skamma stund verður hönd höggi fegin
> a short while becomes the hand eager for hewing???

"For a short while (only) is the hand happy about the blow (it has
truck)." The second time this saying has cropped up in Njáls saga (see
Ch. 42). The Complete Sagas of the Icelanders has a nice ringing
translation complete with alliteration: "the hand's joy in the blow is
brief" [ http://www.usask.ca/english/icelanders/proverbs_BNS.html ].
It's always worth a look at that page when we get to proverbs.

'skamma' f.acc.sg. "short" (agreeing with 'stund').
'skamma stund' "for a short while".
'verður' "becomes" (but in idiomatic English I'd use "is").
'höggi' (older 'höggvi') n.dat.sg. "a blow (struck)" (the dative
indicates what one is happy with/about/over).
'fegin' f.nom.sg. "glad" (agreeing with 'hönd').

> Hefir hann stórt af gert við yður
>
> He has done very much against? you

That's right: he has wronged you greatly; he has seriously
transgressed against you. For the phrasal verb 'gera/gør(v)a af', see
Zoega (13) "with preps." 'gør(v)a af við e-n' "to wrong a person,
transgress against one."