> Á öndverðum öðrum vetri þá komu Skrælingjar til móts við þá og voru

miklu fleiri en fyrr og höfðu slíkan varnað sem fyrr.

> Around (the beginning?) of their second winter, the Skræling came to

visit and in greater numbers than they were before and had the same
sort of goods/merchandise

That's right: "early in the(ir) second winter", 'öndverðr' being an
adjective that agrees with the noun representing the time period. The
opposite is 'ofanverðr' "late", "in the later part of" (e.g. á
ofanverðum dögum Ketils; ofanverða nótt).

In Old Norse, as in Old English, time is visualised as a vertical line
with events approached from beneath, early events lower and later
events higher, thus 'øfri'/'efri' means "higher" and "later". Think of
the High Middle Ages = the Late Middle Ages. Chinese models time
vertically too, but in the opposite direction, which I guess explains
why the Vikings never met up with any Chinese people on their
travels... But only the Aymara of South America have the sense to
picture time backwards, with the future coming from behind where you
can't see it.