Einarr kvað sér eigi mundu svá meingefit at ríða þeim hesti,

Einarr - declared - himself - not - should - so - maliciously inclined - to - ride - this – horse

                                                                                                                                - that – “this” would be þessum      

Einar said that he would not be so ill-minded as to ride this horse,

er honum var bannat, ef þó væri mõrg õnnur til.

which - to him - was - forbidden - if - yet - were - many - others – towards

                                                                                                              - “in addition” (adv) or “to (ride)” 

which was forbidden to him, especially if there were many others to use.

Einarr ferr nú heim eptir klæðum sínum ok flytr heim á Aðalból.

Einarr - goes - now - home - after - clothing - his - and - removed - home - to – Aðalból

                                                                                                carries (them) back to – 3^rd pers sg pres of flytja

Einar then went home to get his clothing and moved over to Adalbol.

Síðan var fœrt (foert) í sel fram í Hrafnkelsdal,

afterwards - was - brought - within - shieling - forward - in - Hrafnkelsdalr

After that (the sheep) were driven to a shieling in the upper part of Hrafnkel's Valley, 

            Question: How do you know the sheep is the subject here? I thought it could be “He (ie Einarr) was brought to the shieling…”

Einari ferr allvel at um sumarit,

Einarr - goes - very well - concerning - during - the summer

            To Einarr (it) goes very well (Einari is dat sg.)

Einar fared very well at it during the summer,

Þat var einn morgin, at Einarr gekk út snimma,

it - was - one - morning - that - Einarr - goes - out – early

                                                                        -went- (gekk is 3^rd pers sg preterite)

Einar went out early one morning,

Hon fell fyrir framan selit.

it - flows - in front of - forward - the shieling

I think that the river simply flowed “in front of” (fyrir framan) the shieling.

            which flowed down from the shieling.

En þar á eyrunum lá fé þat, er heima hafði verit um kveldit.

and - there - on - the gravel banks - lie - livestock - which - at home - had - been - during - the evening

                                                                 - lay (lá is preterite of liggja)

There, lying on the gravel banks, were the sheep that had been at home during the evening.

en ferr at leita hins, er vant var áðr.

and - went - to - seek - them - who - missing - were – earlier (already)

                 -goes- 3^rd pers sg pres of fara             the others (gen sg of hinn, used collectively as you note)

and went to look for the ones who were missing earlier.

ok hugsar at hõndla sér hross nõkkurt til reiðar ok þóttisk vita,

and - considers - to - seize - himself - a horse - certain/any - for - service - and - thinking - to know

                                                                                                possibly: it seemed (to him) a sign (with vita being acc sg of viti (m) see Zoega)

and decided to catch one of the horses to use, believing  

Ok er hann kom til hrossanna, þá elti hann þau,

and - when - he - comes - up to - the horses - then - chases - he – them

                                     - came (3^rd pers sg preterite of koma) –chased- elti is 3^rd pers sg preterite of elta, present would be eltr, I think.

So when he reached the horses, he chased them,