> hvort nokkur er sá
> (whether there is anyone)

That's how I'd translate it. It's that old curiosity of the
demonstrative pronoun being used indefinitely: "Demonstratives do not
necessarily give the N[oun] P[hrase] a unique or specific reference.
[...] A noun modified by a relitive clause may have an indefinite
reading even if it is combined with a demonstrative" (Faarlund 2004,
pp. 85-86). Among his examples is: er nokkurr sá maðr á þingi, er þat
kunni at segja "is there any man at the assembly who can tell us that?"

> Hversu veiti eg yður þess að málum að yður sé skapfelldlegast?

`þess...að yður sé skapfelldlegast' "in such a way that it [my
support] is most to your liking". I'm not sure if this is quite the
same, but I womder if it's related to the way the genitive is used to
introduce a clause expressing the content of an oath: ok sverir þess
eiða at þér haldið eigi orustu heðan í frá í gegn mér "and swear an
oath that you will not henceforward fight a battle against me"; ok til
fjörlausnar vann hann Geirröði þess eiða at hann skyldi koma Þór í
Geirröðargarða svá at hann hefði hvárki hamarinn né megingjarðar "and
to save himself, he swore an oath to Geirrod that he'd bring Thor to
Geirrodargardar without his hammer or belt of strength".

> Flosi mælti: "Gangi (I think this is a 3rd pers imperative, rather

than 1st or 2nd pers) nú allir til mín og sverji (3rd pers imperative)
eiða að engi skerist úr þessu máli."

> Flosi spoke:'Now (let) everyone (all) go to me and (let them) swear

oaths that (they) not withdraw (stray) out of (from) this case.'

Yes – that's to say the 3rd person plural subjunctive serving as
imperative/optative.

> Þórðar illuga

The (nick)name `illugi' probably meant "the ill-tempered" (illr + hugr).

> sá skal hver hafa fyrirgert fé og fjörvi (fjör) er (rel pron rather

than conj here, I think) úr þessu máli gengur fyrr en yfir lýkur.

That's right, I think: sá...hver...er "whoever". We'd tend to put the
relative clause first in English: "Whoever forsakes this case before
it's over will have forfeited his property and his life." Compare
these two examples from Hómilíubók: algjör er sá hver, er heilagur er
"whoever is holy is perfect" (Hómilíubók); nú sá hver, er einfaldur
er, sá mun fályndur vera "Now whoever is simple (innocent) will be
steadfast/faithful".

Regarding the interposition of `skal' here, we met another example of
the constituents of a phrase being split by a verb in Groenlendinga
saga: hin sama var umræða á Vínlandsför sem fyrr "There was the same
discussion about travel to Vinland as before." Some more examples of
this phenomenon: en væta var á mikil um daginn `it rained heavily that
day' (Heimskringla: Óláfs saga helga 112); góðan eigum vér konung `we
have a good king' (Heimskringla: Óláfs saga helga 208). It's
especially common with qualtifiers: ok margir hétu aðrir Glúmi sínu
liði `and many others promised their help to Glum' (Víga-Glúms saga
27); miklu skal þat betra vera áðr en ek létta `it will be much better
before I am finished' (Þiðreks saga 67). Some of these examples are
from Eiríkur Rögnvaldsson 1993, `Þættir úr sögulegri setningafræði' [
http://www3.hi.is/~eirikur/syntsag.pdf ].

> því að einhver mun fyrir þurfa að vera málinu

"because someone will need to be in charge of the case" (lit. because
someone will IN-CHARGE need to be OF-THE-CASE).

`einhver' is nominative, the subject of `mun' (acc. would be
einhvern). For `vera fyrir', see also Zoega `fyrir', dat. 6.

(6) before, at the head of, over; vera f. máli, to be over the troops;
vera f. máli, to lead the case; sitja f. svörum, to undertake the defence;

This is an example of a syntactical quirk sometimes found in Old Norse
whereby a prepositional phrase is split by other words coming between
the preposition (fyrir) and its complement (málinu). Some more
examples: þá mun ek eptir leita þeim eignum ok óðulum, er... `then I
will seek after the properties and inheritances that...' (Óláfs saga
helga 100); var mikit til aflat þessar veizlu `much had been provided
for this feast' (Laxdoela saga 27). For more on this, see Faarlund
2004, The Syntax of Old Norse. Oxford: OUP, § 8.4.2, and Eiríkur
Rögnvaldsson 1995, `Old Icelandic: A non-configuratiuonal language?',
NOWELE 26:3-29 [ http://www.hi.is/%7Eeirikur/ ].