> býzk: (line 635) Is this 3rd pers sg of bjóðask?

Yes, I think so. t, ð > z before s of the reflexive suffix. The
3rd sg. ind. of búask would be býsk. The same change happens with
the 2nd pl. ending -ð + sk > zk. Cf. valzligr "proud" = valdsligr,
vaz "of water" = vatns.

> þurfu: (line 640) Why is this 3rd person plural when ‘hvers’ is

singular?


I don't know. Could it be another example of grammatical looseness?

> vant er við vándum at sjá: (lines 643-4)

"It's hard to be on your guard against the wicked."

vandr "difficult"
vándr "bad, evil" (here in dat.pl. "wicked people")
sjá við "guard oneself (against), be on your guard (against)"

[ http://www.usask.ca/english/icelanders/proverbs_HKLS.html ].

> illt: (line 652) Is this behaving as an adverb or noun? ie ‘much

has arisen badly from him’ or ‘much badness has arisen from him’?

I would just take it as the adjective. The singular is often used
with 'margr' in a plural meaning: 'margt' "much, many
things"; 'margr maðr' "many (people/men)".