vinsælli: (ll. 686, 688) I presume this is the comparative form.

sem mest skip (l. 678), sem mest land (l. 679): What function is sem performing in these phrases?

Vinsælli seemed to me to be comparing present with past behaviour and what else could it have been but comparative 

Sem in both cases appeared (to me) to be serving - "as if" or "it seemed" and might possibly have been a kind of scholarly comment of the Scribe at the time, implying that he was not actually there at the time, but it was stated "as far as he knew - or believed"  - or indeed it was as he had been told "from a well-authenticated source"

Kveðja

Patricia