Here is my best:

Skarphéðinn gekk þá til Gríms bróður síns.
Skarphéðinn then walked to Grímr, his brother.

Héldust þeir þá í hendur og tróðu eldinn.
They hold themselves (mutually) then in (their) hands and walked
(tráða ~ tread ?) (in, surrounded by) the fire.

En er þeir komu í miðjan skálann (skáli ; acc., because of a sense of
motion I think) þá féll Grímur dauður niður.
But when they came in the middle of the sleeping-hall then fell Grímr down dead.

Skarphéðinn gekk þá til enda hússins. Þá varð brestur mikill.
Skarphéðinn then walked to the extremity of the house. Then happened a
big crash/loud noise.

Reið (past of ríða ; I guess the nearest sense is: Z(3): to move
through the air) þá ofan öll þekjan.
Then all the roof went down.

Varð Skarphéðinn þá þar í millum og gaflhlaðsins. Mátti hann þaðan
hvergi hrærast.
Then Skarphéðinn happened to be there between [the roof] and the
gable-end. Therefrom could he move himself nowhere.

Þeir Flosi voru við (close to ; see Z(II.1)) eldana (acc. triggered by
'við' + definite article) þar
(They) Flosi (and his men) stayed there close to the fires

til er morgnað var mjög (Z(3): almost, very nearly). Þá kom þar maður
einn ríðandi að (Z(1) towards, against) þeim.
till when (it) was almost become the morning. Then came there some man
riding toward them.

Flosi spurði þann að (prep. trigerring dative ; s. e-n at nafni, to
ask one his name (Z, spyrja)) nafni en hann nefndist Geirmundur og
kveðst vera
Flosi asked this (man) his name and he called himself Geirmundr and
said (about himself) to be

frændi Sigfússona "þér hafið mikið stórvirki unnið (past. part. vinna
'perform, do' Z(3))," segir hann.
kinsman of the sons of Sigfúss "You (plural) have accomplished great
feats," says him.

Flosi svarar: "Bæði munu menn þetta kalla stórvirki og illvirki. En þó
má nú ekki að (n. an incited conflict or fight (Z)) hafa."
Flosi answers: "Both shall men call this great feats and evildoing.
But now there can't be any incited fight now, though (?). (No one will
avenge this.)"

"Hversu margt hefir (3rd person singular, agreeing with margt ?) hér
fyrirmanna látist (past part. of láta-st, 'to die, perish'; láta,
Z(13)) ?" segir Geirmundur.
"How many (of) chiefs have here perished ?"

Flosi svarar: "Hér hefir* látist Njáll og Bergþóra og synir þeirra
allir, Þórður Kárason og Kári Sölmundarson, Þórður leysingi.
Flosi answers: "Here have perished Njáll and Berþóra and all their
sons, Þórðr son of Kári and Kári son of Sölmundr, (and) Þórðr the
freedman
* 3rd person singular, but why ? Is it in reference with the singular
in the question ?

En þá vitum vér ógjörla um fleiri menn þá er oss eru ókunnari."
And we know them (to be) hardly (?) to most of the menn that are less
known than us. (???)"

Geirmundur mælti: "Dauðan segir þú þann nú er vér höfum hjalað við í morgun."
Geirmundr spoke: "Now you say dead that (man) we (plural) have talked
with in the morning."

"Hver er sá?" segir Flosi.
"Who is that?" says Flosi.

"Kára Sölmundarson fundum (finna; Z(2): 'to meet') við Bárður búi
minn," segir Geirmundur,
"With Bárðr my neighbour we met Kári son of Sölmundr," says Geirmundr,

"og fékk (fá; Z(6): 'to give,...') Bárður honum hest sinn og var
brunnið af honum hárið og svo klæðin."
"and Bárðr gave him his horse and was burned of him the hair and so
the cloth(es). (his hair and clothes were burned)"

"Hafði hann nokkuð vopna (modern form of vápn, I guess) ?" segir Flosi.
"Had he got some (of) weapon(s)?" says Flosi.

"Hafði hann sverðið Fjörsváfni," segir Geirmundur,
"He had the sword Fjörsváfni (is it the name of the sword ?)," says Geirmundr,

"og var blánaður annar eggteinninn og sögðum við Bárður
"and the iron-edge was black (because of the fire) and (we) told
(thought) with Bárðr"

að dignað mundi hafa en hann svaraði því (svara takes the dative) að
hann skyldi herða í blóði Sigfússona eða annarra brennumanna."
that he had lost his temper/heart when he answered this that he should
press/clench (make) [in/to] blood (of) the sons of Sigfúss and of the
other men of the burning.
if I'm right, it means he wants to reduce those men to blood, with the
idea of literally crushing them in his hands.

Flosi mælti: "Hvað sagði hann til (Z(2): of) Skarphéðins?"
Flosi spoke: "What has he said of Skarphéðinn?"

Geirmundur svarar: "Á lífi sagði hann þá Grím báða þá er þeir skildu
en þó kvað hann þá nú mundu dauða."
Geirmundr answers: "He said Grím (and Skarphéðinn) both alive when
they should (when the time is come ?) and though he said that they
were probably dead by now."
I'm really not sure about my translation here, but I guess the meaning
is that Grím and Skarphéðinn are both 'alive' through Kári, as he
promised to avenge them.

Flosi mælti: "Sagt hefir þú oss þá sögu er oss mun eigi setugrið bjóða
því að sá maður hefir nú á braut komist er næst gengur
Flosi spoke: "You have said us then (now) stories that we won't be
offered truce/peace because that man is now gone away and next (it)
comes"

Gunnari að Hlíðarenda um alla hluti. Skuluð þér það nú og hugsa
Sigfússynir og aðrir vorir menn að svo mikið eftirmál mun hér
to Gunnarr of Hlíðarendi of all things (the things will happen like
with Gunnarr ?). You must now think and the sons of Sigfúss and other
<?> men that such so big lawsuit will now

verða um brennu þessa að margan mun það gera höfuðlausan en sumir munu
ganga frá öllu fénu. Grunar (gruna: Z(4) impers., I suspect, doubt.)
mig nú það að engi yðvar
happen from this burning that it will make many lose their head, when
some will go without possession. I doubt that (any of)

Sigfússona þori (þora, subj.: to dare, have the courage...) að sitja í
búi sínu og er það rétt að vonum. Vil eg nú bjóða yður öllum austur
til mín og láta eitt ganga yfir oss alla."
the sons of Sigfúss will have the courage to stay in his farm and that
is a good advice <not to stay there> (?). I want to ofer you all <to
go> to the east to me (to my farm) and let it / one go over us all
(let the things calm down.)