I've just been given some notes from the Icelandic schools edition of
Njáls saga. It defines 'rúm' as "bil milli þóftna í báti", the space
between the thwarts in a boat ('þóftur' "thwarts" are the
benches/boards where the rowers sit). The Oxford English Dictionary
actually gives this exact sense as a rare localised meaning of "room",
"9.d. The space between the thwarts of a boat. (A Scandinavian
sense.)" So used, apparently, in Norfolk, in the 19th century at least.

On 'fengsöm', the notes just say "fengsæl, ötul um aðdrætti", she was
good at getting provisions, or (pro)active in acquiring provisions.
Unlike the English words "acquisitive" or "grasping", it seems that
'fengsamur' doesn't necessarily have a negative connotation. In fact
some of the quotes I appear to use it as an outright positive quality
(fengsöm og staðföst vinum sínum; fengsöm og forvitra). So the author
may be using it here in a tongue in cheek way here.

Regarding the line

"Hann var að hlaða skútuna en þeir báru á út ..."

it's been brought to my attention that the accepted text is a little
different; the schools edition and the Íslensk Fornrit edition both have:

"Hann var að að hlaða skútuna en þeir báru á út ..."

In this version there is an extra (stressed) 'að' before the
infinitive marker 'að'. It doesn't affect the meaning much, except to
emphasise the idea that that he's "busy loading", "in the middle of
loading", "in the process of loading".

Re. 'á út', I've been advised by a native Icelandic speaker that the
sense is indeed "out onto [the boat]". 'út' suggests that the boat is
in the water, so they have to go 'út' "out" to it from the shore to
load the goods 'á' "onto" it. Some quotes from other sagas, with a
preposition before 'út' either with a noun complement or without:

hann af skafti og skaut skaftinu á sjó út, en vopnin
batt hann í bagga í

Tóku þeir til segls og sigldu á haf út. Tók þá byrinn
að vaxa og gerði veður

En ekki nenni eg draga hann úr húsum út." "Þú munt
ráða að sinni," sagði

varð fyrir þeim. Þar hljóp Kýlan á út en Þórir skaut
eftir honum spjótinu því

Lagið kom í skjöldinn og renndi af út og kom á nára
hestinum og þar á hol.

I've been told that it's more natural to put the preposition before
'út' when there's not noun.

As for the whole middle voice / myðmynd thing, I'm not sure of the
best terminology, clearly different writers use different terms (I've
also seen "reflexive middle" used in modern syntactic papers on
Icelandic). I suppose there is potential for ambiguity in all these
terms, but hopefully the context will make things clear most of the
time. For now I'll try to remember to specify "middle voice FORM" or
some such, if I think there's any doubt. And if anyone's confused
they can ask. In case anyone read message 6806, "Icelanders take
Miðmynd for verbs forms suffixed with -ST and in modern Icelandic all
of them I know give passive result", I'll just point out that the
reflexive suffix doesn't always correspond to an English passive:

dásk at e-u / e-m "to admire" (Modern: dást að)
eignask "to acquire" (Modern: eignast)

...and that passive meaning can be expressed in other ways in Old
Norse / Icelandic besides the reflexive suffix: 'hann var drepinn' "he
was slain"; 'honum var(ð) borgit' "he was saved".