> Ósvífurssynir fóru utan það sumar og kom engi þeirra út
> síðan.

> The sons of Osvifr went abroad that summer and they didn't
> come (back) to Iceland after that. (Z. út 3 - koma út, to
> come to Iceland; also of other countries)

> Osvif’s sons sailed abroad that summer and nothing came of
> them – in Iceland- after that.

> (The) sons of Ósvífr went abroad that summer and not-one
> (masc sg rather than neut pl I think) of them came
> back-to-Iceland (lit: out, út) after-that.

I agree that <engi> is masc. sing. nom.

> Lauk þar eftirmáli að Ólafur þótti hafa vaxið af því að
> hann lét þar með beini ganga er maklegast var, þar er þeir
> voru Ósvífurssynir, en hlífði Bolla fyrir frændsemis
> sakir.

> It was brought to an end there, to the action on behalf of
> the Kjartan's death, that Olafr was thought to have grown
> in fame because he dealt there blows to the very bone he
> who was most guilty, there where they were the sons of
> Osvifr, but spared Bolli for the sake of their kinship.
> (Z. bein 1 - láta með beini ganga, to deal blows to the
> very bone)

> The action on behalf of a person slain ended that Olaf
> seemed to have grown from it that he caused there to go
> with help where (it) was most fitting, there since they
> were Osvif’s sons, but helped Bolli for the sake of
> kinship.

> (It) was-concluded there in (the) actions-against the
> slayers (ie the outcome of the case was) that Ólafr seemed
> to have grown (in standing) from that, that (ie because)
> he dealt harshly (lit: caused to go with/ through? (the)
> bone, see bein, Z1) there which was most-fitting, there
> where (ie when) they were Ósvífr’s-sons, but spared Bolli
> for reasons of kinship.

Impersonal <lúka> takes a dative subject, so I'd go with
Grace's translation of <Lauk þar eftirmáli að>, or something
similar. <Með> is probably best translated 'to'.

[The] action against the killers ended that Ólaf was
thought to have grown in fame because he dealt blows to
the very bone there where [they were] most deserved, there
where they, Ósvifr's sons, were, but dealt gently with
Bolli for kinship's sake.

Brian