Re:  … honum þótti þat líkast í hýbýlum Guðmundr skáld og hundr sá, er Rósta hét …

>> … that seemed to him most-likely[that] Guðmundr [the] scaldic-poet and that dog, which was-called Rósta [were] in [the] homestead …

>… he thought Guðmund [the] skald most like [amongst those] in [the] household the dog that was called Rósta [‘brawl, riot, tumult’] ...

To me <líkast> looks neut nom sg, so therefore it must be agreeing with <þat> rather than <Guðmundr>. For clarification, are you saying that the literal reading of the sentence is:

“That seemed to him the-greatest-likeness (resemblance) in the household, Guðmundr the poet and the dog who was called Rósta,” where Þat = Guðmundr + Rósta

I agree that that makes more sense in the context of what follows.

Alan