> Sáttgjarn hefi eg verið jafnan
> eager-of-agreement (conciliatory) ...

Yes: "I've always wanted (been keen/ready/prepared/happy/glad to) to
come to an agreement", "...to settle this thing peacefully."

> eigið þér nú eftir mikið að mæla

Zoega: mæla eptir e-n "to take up the prosecution in the case of a
slain man" (þú átt eptir hraistan mann at mæla).

So: you have a lot to bring to court; you have have a serious case to
make; you have grievances indeed to state; MM & HP "the grievances for
which you seek redress are heavy".

> Hann var sonur Barkar ... Synir þeirra Starkaðar og Hallberu voru

þeir Þorgeir og Börkur

> He was (the) son of Bark ... were they, Þorgeir and Börk

Barkar is the genitive of Börkr.

LN