> leggja utan við Agðanes
> put-in from abroad at Agðanes

Yes: 'utan' from abroad (conventionally with respect to Norway, even
when talking about people who's home is Iceland) = 'utan af Íslandi'
from Iceland.

> Guðbrandur mælti: "Hér munt þú vera hljóta."
> Guðbrand spoke: 'You will be obliged to (stay) here.'

In this context, I think it's more like: "You will get to stay here."
(See Zoega's first two definitions.) In other words: "You can stay
here" rather than "you'll have to."

> "Á hinn óæðra bekk," segir Guðbrandur, "gegnt öndvegi mínu."
> 'At the lower-rank bench,' says Guðbrand, `opposite my high-seat.'

The place he's alloted, 'öndvegi it óoeoeðra', is the traditional seat
of honour for guests: the most prestigious seat after the lord's seat
'öndvegi it oeðra'. See the diagram in Gordon's Introduction to Old
Norse, p. 229. So the idea is that he's treated with great respect by
his host but doesn't respond in kind.