> Búi sagði honum allt hið sanna.

> Bui told him all the truth.

> Bui told him all the truth.

> Búi said to him all that (neut) truly (note: sannr, adj,
> Z1) (ie the complete truth).

I'd go with 'all the truth': <hið sanna> appears to be
elliptical, 'the true [information]', more or less
equivalent to <hinn sann> (masc. accus.) with the
corresponding noun.

> Skeiðhlíð (Good?-Slope (for riding on), cf skeið-gata,
> skeið-reitt).

It might simply be a hill that was part of a racecourse;
I've not found any other mention of it.

> Búi gekk upp á hólinn og bar upp grjót að sér.

> Bui went up the hill and carried up stones for himself.

> Bui went up on the hill and collected up stones for
> himself.

> Búi walked up onto the-knoll and carried up stones for
> himself (for his use?).

I'm not sure whether it's 'for his use' or expressing that
he did the carrying himself.

> Hlupu þeir þegar af hestum sínum og veittu Búa atsókn
> harða.

> They hopped at once from their horses and Bua very helped
> attack. (?)

> They leaped from their horses at once and gave Bui a hard
> attack.

> They leapt at-once from their horses and attacked (lit:
> gave an onslaught) Búi fiercely.

Literally 'gave a hard onslaught'.

> Var það jafnsnemma að fallnir voru fjórir menn af þeim
> bræðrum enda hafði Búi þá lokið grjótinu.

> That was at the very moment that four men fell from the
> brothers although Bui had finished up the rocks.

> It was at the very same time that four men were killed of
> those brothers and Bui had then finished (ran out of) the
> stones.

> That was at-that-very-same-moment that were (had) fallen
> (been killed) four men (persons) from those brothers
> and-indeed Búi had then finished (ie run out of)
> the-stones.

He's lethal with those stones!

> Og með því að þeir bræður voru sárir, Helgi og Vakur, þá
> gáfu þeir upp.

> And because the three brothers, Helgi and Vakur, were
> wounded, they gave themselves up.

> And since those brothers were wounded, Helgi and Vak, then
> they gave up.

> And with that (ie because) those brothers were wounded,
> Helgi and Vakr, they then gave up (not strictly
> surrendered or ‘gave themselves up’, I think they just
> stopped fighting).

I agree; if it were 'surrendered', I think that it would
have the reflexive pronoun.

Brian