I'm getting a bit behind you all this week...

> haldi þér kost:(I´m not quite sure how to render 'kost' but suspect

it may also hint at double-meanings.)

en móðir mín haldi þér kost
"My mother will take care of you" (MM & HP). Or since it's the
subjunctive, maybe you could say "let my mother..."

> og ger þeim þar góða veislu

MM & HP: "and prepare a fine feast for them there"

> og fylgdi hann þeim í eina steinhöll

MM & HP: "...ushered..."

> Nú mun það sannast er eg sagði þér frá Gunnhildi.
> Now will that (be shown to be) most true which I said to you about

Gunnhild.

I think 'sannast' = 'sannask' "prove true, be show/found true", the
middle voice of the verb 'sanna', rather than the superlative
adjective "most true".

> þá er þú ert í boði mínu.

MM & HP: "as long as you are my guest". (I think the noun is related
to the verb 'bjóða' "invite", rather than 'biðja' "ask, bid, demand".

> Síðan gengu þau til svefns og læsti hún þegar loftinu innan
> Afterwards they went to `sleep' and she locked the upstairs bedroom

from inside

MM & HP don't translate 'þegar' either: "and she locked the door." If
"at once" sounds not quite right for the context, maybe we could say
something like "without further ado", or "she didn't waste any time
but locked the door" or "no sooner were they there than she locked the
door" or "next thing (he knew), she'd locked the door" or "the moment
they got there, she locked the door", or some such paraphrase?

> Þér skuluð engu fyrir týna nema lífinu
> You shall lose nothing except the (your) life

MM and HP paraphrase differently, but I've a feeling this might
literally be: "You won't lose anything BEFORE (you lose) life", i.e.
"the first thing you'll lose will be your life".

> hundrað álna hafnarvoðar og tólf vararfeldi

MM & HP: "a hundred ells of fine cloth and twelve furs"