> Hún var ákaflega mikil og sterk sem karlar
> She was a very spirited woman (Z-vehement - do not like)

"She was extremely big and as strong as men [are]". This is 'ákeflega'
in Zoega's second sense, "very, exceedingly", just intensifying
'mikil'. The narrator doesn't comment on her personality.

> og lágu þau bæði senn

"and they were both laid up [with it] at the same time"

> og lagðist Grímhildur niður í því
> and Grimhild collapsed down (?to the floor?)

"and at that [no sooner than he came in] G. lay down". 'í því' = 'í
því bili' "at that moment".

> og brakaði þá í hverju tré í stofunni.
> with a creaking of the wood/planks of the room

Yes, "and there was a creaking/cracking then in every timber in the room".

> bjó um

"dealt with it", "laid her to rest"

> I was unable to stop from being much affected by the narrative of

this part of the Saga., and the account of the Kindness shown through
out Ch5 of Thorstein the Black.

It's an interesting shift in the mood isn't it, from grisly,
knockabout horror (although we're not quite through with the eeriness
yet...) to this moment of kindness.