Sæl Sarah!

Excellent translation. I'll give you some hints
on the part that caused you troubles.

> Þrándr var ok með því móti þá er hann þroskaðisk,
> en miseldri þeira bræðra var mikit.
> OK. I lost the plot here! I decided some of it
> must be idiomatic use of language - good excuse!!
> I worked out that því is dative singular of sá [this]

Yes, it's idiomatic - which explains everything and nothing :-)

I often have this problem too with languages I'm learning.
I seem to understand all (or most) of the individual words
in the sentence, yet I have no idea what the sentence means.
Can be very frustrating.

Someone learning English might know the meaning of 'take' and
that of 'place' yet have no idea what the idiomatic expression
'take place' means. Someone learning Icelandic might know that
'nema' means "take" or "study" and that 'staðr' means "place"
and she wouldn't have a clue that 'nema staðar' means 'stop moving'.

> þá er hann þroskaðisk - is when he prospered??

Close.

> mis - seems to be a suffix like the English mis-
> eldri - can be the comparative of gamall!!

It's actually a noun. And you're right about mis being a prefix.

> þeira bræðra var mikit is - of their brothers was great??

Keep in mind the idiomatic usage of the pronoun hann (et alia).
I can say things like:

'Vit broeðr' = "My brother and I".
'Þau Guðrún' = "Guðrún and he".
'Þeir Eyjólfr' = "Eyjólfr & co.".

Kveðja,
Haukur