> Þeir ríða leið sína upp til Sópandaskarðs og yfir
> Langavatnsdal og svo yfir Borgarfjörð þveran.

> They ride their way up to Sopandaskard and beyond
> Langavatndale and so beyond Borgarfiord crossroads (?).

> They ride on their way up to Sopanda Pass and over Long
> Water Dale also over across Borgar Firth.

> They ride their way up toward Sópandaskarðr (Sweeping
> Mountain-Pass) and over Langavatnsdalr and so over
> (passed) Borgarfjörðr (Dome-shaped-hill-Fjord) cross-wise
> (ie the fjord´s breadth rather than its length).

My other edition explains that Sópandaskarð is a pass
between Langavatnsdal and Hörðudal, and that Langavatnsdal
is surrounded by the mountains that divide the valleys on
the Breiðifjörð from the Borgarfjörð region to the south.
There's a picture of the path up Skópandaskarð at
<http://www.environice.is/images/Mynd_0399427.jpg>.

> Riðu þeir Reykjardal og svo yfir hálsinn til Skorradals og
> svo upp eftir skóginum í nánd bænum að Vatnshorni, stíga
> þar af hestum sínum.

> They rode (to) Reyjardale and so beyond the ridge to
> Skorradale and so up along the forest in the neighborhood
> of the farms to Vatnshorni, (they) get of their horses
> there.

> They rode (through) Reykjardal and so over the ridge to
> Skorradal and so up along the forest in the neighborhood
> of a farm at Waterbend?, dismount there from their horses.

> They rode (the length of) Reykjardalr (Dale of Steam) and
> so over the-ridge towards Skorradalr (Rift?-Dale) and so
> up along the-wood in proximity to the-farm at Vatnshorn
> (Lakes-corner), there step off (dismount) their horses.

<Vatnshorn> is named for its situation at the southeastern
tip of a lake. After looking at the map at
<http://www.viewweather.com/w2825381-weather-forecast-for-vatnshorn-west.html>,
I think it likely that in this place-namd <-horn> means
'horn-shaped'.

> Förunautar Þorgils báðu hann fyrir sjá.
> Thorgils' companions asked him to take care of (it).
> Thorgils comrades bade him watch out for himself.
> Þorgil’s travelling-companions bade him to foresee.

I'm with Rob on <fyrir sjá>: they told him to see to it, to
go ahead as he had outlined.

> Gerir Þorgils nú klæðaskipti, steypir af sér kápu blárri
> en tók yfir sig voskufl einn grán.

> Thorgils now makes a change of clothes,

> Thorgils now makes a change of clothing, takes off a blue
> cloak and put on a grey rain cloak.

> Þorgils makes now a change-of-clothes (puts on a
> disguise?) casts off him (his) blue-black cloak but (and)
> put over himself a certain grey rain-cloak.

He was apparently dressing down to disguise his rank: the
blue cloak, unlike the rain cloak, was something that only a
wealthy, important person would have worn.

> Maðurinn svarar: "Þú munt vera furðu heimskur maður og
> fávís ef þú hefir eigi heyrt getið Helga Harðbeinssonar,
> hins mesta garps og mikilmennis."

> The man says: "You would be an astonishment, a stupid and
> silly man if you have not been able to hear of Helgi
> Hardbeinson, the most bold and powerful man."

> The man answers, “You will be a strange, silly man and
> foolish if you have not heard tell of Helgi Hardbein’s
> son, the most bold and powerful man.”

> The man (person) answers: “You will be a very-foolish man
> (person) and of-little-wisdom if you have not heard spoken
> of (I don´t think this is a case of geta + pp, as I don´t
> think heyra e-s would be a valid syntax) Helgi
> Harðbeinn’s-son, the most dauntless-man and man-of-power.”

I think that you're right about <heyra e-s>. The one
example that I've found without a preposition uses the
accusative:

Hvárt er at þú ert maðr svá óvitr at þú hefir eigi heyrt
ófrið þann er í milli er landa þessa, eða ...

Brian