> Þik skal leiða ór garði brott ok
> þá menn, sem mér líkar, ok VERA drepinn.
>
> I shall lead you out of the farmyard, away,
> and those men, whom it please me, and to be killed.
>
> What is the grammatical subject of this verb?

I think of it as if there's a [þú skalt] omitted.

"Sá annarr at þik skal leiða ór garði brott
-ok þá menn sem mér líkar- ok [þú skalt] vera
drepinn."

Since "þik skalt" has already been stated the author
didn't feel any need to repeat it as "þú skalt" even
if that makes it hard to parse in schoolbook grammar :)

> því at lítil mun vera uppreist
> þeira, ef ek dey FRÁ.
>
> because their success will be
> little, if I die from (this).’
>
> What function is this performing?

It's just a phrasal verb; "deyja frá" - "die away".

> Sámr skipti Hrafnkeli af fé slíkt,
> er hann vildi, ok var þat (´took off
> Hrafnkel’ or ’apportioned to Hrafnkel’?)

I'd say aportioned (or "divided").

> Þorgeirr mælti þá við Sám: 'Eigi veit ek,
> hví þú gerir (indicative or subjunctive?) þetta.

Indicative. I'd translate: "I don't know
why you're doing this."

Kveðja,
Haukur