> "Vissir þú eigi," segir Kári, "hver (1) fyrir varð?"
> "Did you know not," says Kári, "who in-front happened-to-be?"

> 1- Why hver and not hverr ?

This is just because the text we're using follows Modern Icelandic orthographic conventions. Where inflectional `r' in Old Icelandic followed directly after a root that ended in `r' (as in `hverr' "who", `herr' "army", `ferr' "goes", `spyrr' "asks", `dyrr' "door(s)", `berr' "naked, bare"), the equivalent form in Modern Icelandic simplifies the consonant cluster to a single `r', thus: `hver', `her', `fer', `spyr', dyr', `ber'. But where the root of the word ended in double `r', both are preserved in Modern Icelandic, thus: `verr' "worse".

> "Líkt þótti mér vera Þorsteini bróðursyni (2) Flosa,"
> "Likely seemed to-me to be Þorsteinn, nephew of Flosi,"

> 2- Dative (governed by fyrir?)

No, the fact that `fyrir' takes dative is a coincidence. The name Þorsteinn is dative here because Þorsteinn is the complement of the adjective `líkt'. Some more examples:

Kári er øngum manni líkr
"Kári is like nobody" (that is to say, Kári has no match, Kári is unrivalled, nobody is as good as Kári)

Er þar mikill maður á baki í bláum klæðum, og sýnist mér líkt Hrafnkeli goða.
"There is a tall man on horseback, in dark clothes, and it seems to me most likely (to be) Hrafnkell the Chieftain/Priest"

og þótti mér líkt vera gneggi Freyfaxa
"and it seemed to me to be like the neighing of Freyfaxi [which I hear]" (where Freyfaxi is the name of the speaker's horse)

hverju þykkir þér þetta líkt
"what do you think this is like" (what do you think this resembles)

en hann fór niðr í jörðina undan, því líkt sem á vatni væri
"but he sank down into the earth as if it was on water (that he was standing)"

> segir Ingjaldur. "Njót þú heill handa," segir Kári.
> says Ingjaldr. "Good luck go with your hands (3)," says Kári.

> 3- Z's translation (njóta, Z1). I understood it somewhat differently: "May you benefit luck from (the work of) your hands."

I think you've got the right idea. I suppose you could translate it fairly literally as "enjoy lucky hands" or "have the benefit of (possessing) lucky hands". It's idiomatic, which is why translators might want to paraphrase it for the sake of clarity, as Zoega does, rather than rendering it word for word. Kári is saying "well done", expressing his approval for what Ingaldur did.

> Var Mörður þar fyrir með (5) allmiklu liði.
> Mörðr was there at-the-head with (of) a very-great band.

> 5- Is "fyrir" used as an adverb here ?

Yes. In this context, it just means that Mörður was present. He was there (already) when they arrived.

> Þá skiptu þeir leitinni (6).
> Then they split the search.

> 6- Dative triggered by skipta ?

That's right.

> norður allt til sands
> north all (adv.) to the sea shore

"all the way north to Sand" or "north all the way to Sand"

> Þorgeir mælti: "Þann veg er þó að eg
> ridden around. Þorgeirr spoke: "This weigh (I) that even though (~ syntax ?) I
> sé (vera, pres. subj.) ekki mikill höfðingi þá mun Flosi þó annað
> am no great chief then (that) should Flosi [though, yet ... ?] take
> annað ráð taka en ríða fyrir (+ acc. : ZII.1) augu mér þar sem hann
> other advice than to ride in front of (the) eyes to-me (7) there as (Z: "seeing that") he

> 7- The construct must be somewhat similar to: "doing something to someone."

`þann veg er' "it is that way". The accusative of `vegr' "way" is used to make an adverbial phrase, meaning "it's like this", "the situation is like this", "the situation is thus".

`þó að ég sé ekki' "even though I may not be", "even if I'm not"

`þá' here used to introduce a main clause after a subordinate clause that depends on it; "yet, nevertheless, even so"

`mun' "will (probably)"

`annað ráð taka' "take some other course of action", "take a different course of action"

`en ríða fyrir augu mér' "than to ride in front of my eyes" (That is to say: he isn't likely to let himself be seen by me. He'll probably ride a different way and keep out of my sight).

> þar sem

"seeing as/that", "given that". Introduces an explanatory clause.

> hann hefir drepið Njál föðurbróðir (8) minn og sonu hans
> has killed Njáll my uncle and his sons

8- Why the nominative ?

I think it must just be an error in the text we're working from. The accusative is demanded by the context (and `Njál' and `sonu' are accusative, as expected), so it ought to be `föðurbróður', in both Old or Modern Icelandic. Konráð Gíslason and Eiríkur Jónsson's edition has the correct accusative form `föðurbróður' [ http://dp.rastko.net/projects/projectID438a3f4017f9f/projectID438a3f4017f9f_TEI.txt ].

> veri
> be

Yes, "be" or "stay".

> og munu þeir nú vera sem varastir (sem + superl. pp. of vara: be on one's guard) um sig."

most on their guards about you."

"and they will now be as wary as possible". When `sem' is used with a superlative like this, it means "as...as possible", "as...as can be", "extremely...".

> hefðu (subj.)

That's right. Besides the Modern Icelandic spellings, this text we're using has been adapted in some ways to Modern Icelandic morphology. The Old Icelandic form of the preterite subjunctive 3rd person plural was `hefði', the same as the singular, but in Modern Icelandic, the indicative ending is used in the plural, with just the mutated root vowel to show that it's subjunctive.

> vikið

Neuter past participle of `víkja' = Old Icelandic `vikit'.