> "Sjaldan var eg þá (I think this is probably a pronoun rather than
adverb) frá kvaddur er hin góðu voru ráðin."
> `Seldom was I excluded (see kveða, Z3) from them when the plans were
the good (ones).'

That's right, it preempts the subordinate clause: 'þá...er' "when".

> hjá garði nokkurum (I think nökkur, Z3, some, a certain, m dat sg,
agrees with garði).
> beside a certain wall

Yes.