> "Hversu má svo vera?" segir Mörður, "og seg mér enn gerr."
> "How can this be " says Morð "tell me more plain (?) i.e. in more detail

Yes, exactly right: tell me [yet] more fully, more plainly, in more
detail. Magnús Magnússon and Hermann Pálsson: "Be more explicit."
Listed in Zoega as 'gørr', adverb "more fully", related to the
adjective 'görr', 'gørr', 'gerr' "skilled; ready; done".

> eftirlæti
> satisfaction

Yes. Cf. the verb 'láta e-t eptir e-m' "to indulge someone in/with
something, to grant something to some one".

> En þó áður við skiljum sýnir hann það af sér að hann er í æði sínu

rétt sem aðrir menn.

> But however in other ways his nature is as right (normal) as any

other man

Magnús Magnússon and Hermann Pálsson: "And yet, before we draw apart,
he proves that he is by nature as normal as any other man." I took
this to mean that he can and does ejaculate eventually, just not in
her. Is this what other people thought? Note that 'rétt' is neuter,
acting as an adverb "exactly", rather than an adjective describing Hrut.

> Þú skalt vera við hann blíð og eftirmál og mun honum þykja góð

skipan á komin. Þú skalt enga fáleika á þér sýna.
You shall be friendly / compliant (?) with him and all shall seem to
him in good order.
You shall not be - seeming cold in yourselff (cold or sad ?)

MM & HP "You must be affectionate to him and compliant, and he will
think the situation much improved. On no account must you show him
any indifference."

> sem best
> as best they can

That's right.

> sjá fyrir málinu

MM & HP "take care of the case".