> vel að sér

See CV, `at' III.5. "elliptical", half way down the right-hand column
of this page [
http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0027.png ]. It
translates this example "noble-minded".

> Eg er og þess ófús að [...]
> (úfuss, acc? But, if so, why?)

"Besides I have no { wish, inclination} to"
"Besides I am not { keen on, eager to}"

`ófús' is a nominative adjective (the negative prefix `ó' + `fús'
"eager, desirous"), agreeing with `eg'. Accusative would be `ófúsan'.
In Modern Icelandic, whose spelling conventions are followed in this
text, final `ss' is simplified to `s' in the nominative of nouns and
adjectives where the second `s' is the nominative ending (although
double `ss' remains in the genitive). So in normalised Old Norse
orthography, this would be: `Ek em ok þess ófúss at [...]' (or
`úfúss'). The adjective `(ó)fúss' takes a genitive complement for the
thing (not) desired, as is often the case with words for wishing,
needing, requesting, awaiting and the like.

> ef oss dregur undan
> "if we escape"

That's right. Zoega has a couple of examples of this idiom: "d. undan
em, to escape from one (nú lægir seglin þeirra ok draga þeir undan
oss); impers., hann (acc.) dró undan sem nauðuligast, he had a narrow
escape". Note that `oss' in our example is accusative.

> dyrnar (how do you tell if it´s one or more doors?).

Apart from context, I don't know if you can from the grammar alone.