> í jaxlana
> into the lower jaw (?)

Lit. "the molars": jaxl (masc.) + accusative plural ending -a- +
suffixed definite article -na.

> að langt var um ófært
> that for a stretch it was impossible (unfordable?)

'langt um' "quite, totally, completely" (compare English intensifiers
like "far and away", "by a long way", "by miles"). So 'langt um
ófoert' "quite impassable" (by normal means, to any sensible person...
They obviously weren't counting on our friend Skarpheðinn!).

> mein gert
> wounded (?)

Yeah, that's the idea: "done harm (to)". 'mein' is a neuter noun, in
the accusative here.