Given their comparative isolation, were the Norse as open to other gods as the Greco-Roman polytheists, or was their pantheon considered more "absolute" in the vein of the Abrahamic faiths?

Notwithstanding /u/Searocksandtrees, I'm going to give this a go anyway.

There is some evidence that the Norse pantheon accepted Jesus as, if not the absolute God, then a kind of complementary god to their own. Else Rosedahl, in The Vikings, tells us that "arm-rings, neck-rings, necklaces and pendants, sometimes with Christian or pagan symbols, such as a cross or Thor's hammer, were also worn" (page 39). Christians also managed to introduce some of their own values into the Norse societies. For example, leaving unwanted babies in the wilderness to die was common practice at one point, but "Christians reacted strongly against this practice, and it was eventually banned, except in the cases of deformed children" (progress? Page 61).

One thing to keep in mind is that the Northern polytheistic religion didn't, as far as we know, have a holy text and a set of rules that most people followed in the vein of Christianity. There were various 'cults' devoted to different gods for different reasons, like Thor, Odin or Freya. Rosedahl mentions a cult of Odin in Denmark, and there are other indications that the various cults were led by village leaders and chieftains at a local level: "Not much is known about how the gods and other beings were actually worshipped… the cult seems to have been decentralized and led by local chieftains or wealthy farmers. On certain occasions the farmers from a region would meet to honor the gods at a sacrificial feast, or blót. This normally took place in the hall of the chieftain’s farm (a hall of this kind was probably called a hof). There was also private worship in the home" (151). So there was plenty of room for individual variation. Of course, the Scandinavian countries were eventually converted to Christianity, and in at least one case (Iceland) pagans and Christians lived side-by-side for a time with a respect-your-neighbor policy.

So even before conversion, Christians had some influence over how the pagans conducted business, and the decentralized nature of the faith meant that there was likely some mixing going on, as we can guess from the crosses we've dug up in Scandinavian lands. Dan Carlin over at Hardcore History had a theory in his podcast, "Thor's Angels", that when Christians came in contact with Scandinavians, Thor actually influenced Jesus more than the reverse--that an image of a more warlike Jesus became the norm, instead of the original pacifist figure. Can't direct-quote from that, though, since I haven't a transcript in front of me. Hope this helps!