Or were those aspects of Norse mythology an addition that came with increased Christianization, to cast things in a more black-and-white good-versus-evil light?
We don't know.
I'm gonna let you in on an open secret about the early Middle Ages. We dont know anything about the beliefs of the Norse. We cannot name a single tenet/doctrine/guideline for their religious tradition with any real certainty. This is because we count the number of contemporary descriptions of Norse religion that were written down by practitioners on no hands. They simply dont exist. Every single source we have on "Norse mythology" is either a later creation, written after conversion to Christianity, or was written by Christians, almost invariably with no actual first hand knowledge. Trying to base an understanding of their beliefs about the afterlife, cosmology, and so on without primary sources is a little difficult as you might imagine!
All of the hallmarks of Norse mythology we know and love and see repeated in games, movies, books and so on are ultimately derived from sources that arent actually depicting Norse beliefs. Odin as chief of the Gods, valkyries carrying the glorious dead to Valhalla, Loki as a trickster and agent of Ragnarok, and so on, all of this comes from a handful of sources most written in Iceland, centuries after conversion. So why should one small group of sources from one corner of the Norse world stand in for the entire culture across its history across a geographic span from America to Russia and over several centuries?
There never was one single "Norse mythology" that was doctrinally consistent over the Norse/Germanic world temporally or geographically. The stories that Snorri Sturluson edited and compiled into his own works almost certainly were not the same as the stories that held sway in Sweden, or Geatland, or Saxony before its conquest by Charlemagne. Indeed Snorri's own work was compiled centuries after conversion to Christianity in Iceland, long after remnant communities would have stayed pagan. Indeed, even the Eddas are inconsistent on who gets to go to Valhalla or Freyja's Halls, many sources make no mention of Freyja's halls at all. Archaeological evidence is not particularly useful when describing theology, so I don't think it is particularly relevant to bring up here in depth.
This inconsistency in the sources seems to indicate to me at least, and certainly plenty of scholars who have fancy degrees and DO read Old Norse, that there was never any sort of doctrinal coherence to Germanic paganism or Old Norse practice. So this is a roundabout way of saying that while your question is a reasonable one and certainly an interesting one, it unfortunately will likely remain an unanswerable one.
Perhaps it might be best to conclude on an analogy. I do not know your particular religious affiliation, but I'm going to assume that you're roughly familiar with Christianity. Christianity has many things that Norse paganism lacks, such as a single coherent book from which the majority of the religion's theology is derived, and yet any conversation with different denominations or a cursory examination of religious history will show that getting everyone to agree what constitutes Christianity and what the various beliefs of the religion should be is extremely complicated. Hell, theological debates, excommunications, and the like have raged between old and storied Churches over a single word in a prayer. Now take all of those divisions, and remove the Bible as an authoritative source. Now imagine what that might mean for the religion. Indeed, it might seem that it would be all but impossible to construct a religious system with firm answers to a lot of questions in the absence of such a central work. To which I say, welcome to the club.
As for the former two, Fenrir and Surtr, as I mentioned briefly in Did Vikings sacrifice captives/enemies to gods and deities like the fire giant Sutr?, I'd assure we have almost no trustworthy positive evidence from pre-Christian Scandinavia that tell us how they actually regarded such beings out of the extant mythological stuffs.
Virtually no people seemed to 'worship' these two supernatural beings, and it is not so easy to identify the actual existence of the beliefs in Viking Age iconographic sources, such as the carving on the runic or picture stones, than generally assumed.
A few possible examples on the carving illustration on Fenrir have certainly been suggested, scholars have not reached agreement on any identification. The earliest certain allusion to Fenrir is found in the 10th century eulogy poem dedicated to the dead ruler, and the 'wolf' has been depicted as a enemy of the gods, by the mouth of Óðinn himself (Eiríksmál, St. 7; Abram 2010: 97f.). As for another famous episode on Fenrir, that is to say, its raising among the gods and binding by Týr, however, we don't have much sources (especially from pre-Christian period) to support or to negate the description of Eddic poems (the 12th century poems from Christianized Orkney is only non-Eddic sources that deals with the episode. Lindow 2001: 113), so we are not so sure whether Fenrir had indeed been raised among the gods at first, as narrated in Snorri's prose Edda (Gylfaginning). Anyway, even if we accept the established supposition that Fenrir would turn hostile against the gods later in spite of its earlier upbringing among the gods, his relationship to other gods was at most the same as Loki who was neither worshiped almost certainly in pre-Christian Scandinavia (for Loki, please also check /u/Platypuskeeper's elaborated comments in How plausible is the theory that Snorri re-wrote Loki into a Lucifer-like enemy of the gods in order to keep the Norse stories alive in a Christian culture, but Loki was more of a positive trickster in the original mythology?). So, I suppose that overall fragmentary evidences depicts Fenrir primarily only as a one-sided enemy of the gods, not any object of the cultic activity.
We know even fewer details on Surtr than Fenrir (so, what I wrote in the linked thread is almost all I know about him).
On the other hand, Jotnar seemed to have been totally different for the Norse people from these two mythical supernatural beings. While some famous (especially male) giants (Jotnar) take the role of the villain in Eddic-mythological narrative, several modern scholars interpret the giant/ giantess as representative of the chaotic raw power of the nature, in contrast to the gods that symbolize the order (Steinsland 2005: 140f). Gods and giants are two axes of opposing powers in the universe, and both were indispensable. Remember that it was not until the arrival of three giantess daughters in Völuspá (St. 8) that neither the dwarfs nor the human beings had not been created by the gods.
It is well-known that especially some giantess in Old Norse mythological texts involved with the gods either as a possible lover or even consort, mother, but an pre-Christian Norwegian genealogical poem, Háleygjatál, tracks the ancestry of the family of Earl (jarl) of Lade, back to the one-time (?) affair between Óðinn and Skaði on the land of human beings (Háleygjatál, St.2). While the latter was famous as a huntress-goddess and wife of the god Njörd, she had originally been a daughter of the giant. In other words, the poet (Eyvindr) states in the poem that the patron's ancestor was originally a illegitimate son between the god and the giantess.
It is also worth noting that the giants and giantess sometimes played a role of mentor for the young hero in Old Norse literature. The most famous example is probably the case of Hadding in the Deeds of the Danes (Gesta Danorum, c. 1200), written in Saxo Grammaticus. The legendary hero Hadding was born as a son of the king of the Danes, but he was raised by the giantess Harthgrepa, after the slaying of his father by the king of Norway. Another, less known, but more interesting example is found in a later tradition on King Harald Fairhair of Norway who would allegedly conquer and unite all the Norway, calling him as 'Dovrefostri' (fostered by Dovre the giant) (Steinsland 2012: 103-107). In this tradition, young Harald was bad terms with his father, Halfdan, so he delved into Dovre mountain areas, Central Norway, to meet the namesake giant to learn some knowledge useful for the ruler.
Steinsland interprets this episode on young (future) King Harald as following:
'In the mythology of the Viking Ages, it was the giants (Jotnar), not the gods, who ruled in the mountains. The giants were wise and possessed knowledge useful both for the gods and for huma beings, but they represented a dangerous counter-power to the order of the gods. This dangerous 'other world' was a resource that must be ascertained either by the alliance or by the contract with the supernatural beings that ruled it' (Steinsland 2012: 104, the translation is mine).
If we accept her hypothesis, Dovre mountains and their supernatural 'inhabitant', giant, were once regarded as 'holy/ sacred' by Viking Age Norwegians, and they did not cast this kind of beliefs on the natural landscapes away totally after the acceptance of Christianity. The 19th century Norwegian novelist Henrik Ibsen wrote in his famous drama series Peer Gynt that the protagonist (Peer Gynt) rode in the mountain hall in which the troll king resided. This troll king was called 'Old Man of the Dovre mountains (Dovregubben)', so he might have been the ruin of former giant Dovre, though still dangerous......
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