There may be a specific answer to this theological question, but certainly relevant is this great recent answer by u/steelcan909 to a different question that applies to yours as well:
Basically, nobody can definitively say what the pre-Christian Norse really believed— almost everything we know of their beliefs was written down much later by Christians.
Another relevant answer by the same user: https://www.reddit.com/r/AskHistorians/comments/fgulo2/im_a_deceased_warrior_who_fell_in_battle_where_do/?utm_source=share&utm_medium=ios_app&utm_name=iossmf
Answers above are tackling and supplying links in response to the faith/belief parts of this question. The literary answer is that Ragnarok is mainly attested in an early part of the Poetic Edda called Völuspá.
The poem takes the form of a seeress (völva) addressing Odin with a prophecy. Völuspá means the Seeress' Prophecy. It tells the history of the world and its ultimate fate, starting with creation and ending in destruction.Therefore, the literary explanation for 'does Ragnarok still have to happen' but 'how do they know what Thor has to do' is that it's delivered as a prophecy, as part of a simple historical cycle.
It's also worth adding that medieval Icelandic literature, of which the Poetic Edda is a part, strongly dealt in the Christian textual feature called typology. Essentially this meant that events in the New Testament were foreshadowed by the Old Testament. Christian themes could be implemented in medieval texts by typologising Biblical vignettes and having them recur in identifiable ways in the medieval stories. I'm not saying that this is what necessarily happens in the Völuspá, but the idea of historical cyclicality and repetitious events were certainly recognisable to the literary class at the time.
This is quite the flurry of questions, so I’ve taken the liberty of separating them into the two major questions being asked:
Now firstly, whenever we’re dealing with the viking era of early middle ages, it’s always important to keep the following disclaimer in mind as stated by r/steelcan909: “We dont know what we think we know about Norse mythology, and it's impossible to try and extrapolate from the material that we do have to other cultures" So keep this in mind as the following answer will be based on the only source capable of somewhat answering it, which is the origin of the Ragnarok myth itself: The Icelandic Sagas.
What we’re dealing with from a historical perspective when it comes to northern mythology is not a contemporary source, but a 2nd hand source written centuries later in a Christian discourse and thus often affected by it. The only actual contemporary written sources from the viking age time period is either rune stones (usually memorial) or runes written on items, which usually boiled down to a signature as in “I made this sword” or a message on a branch saying “Helga says to come home now” etc. Once again, Steelcan909 said it best: "All of this comes from a handful of sources most written in Iceland, centuries after conversion"
When we’re dealing with Ragnarok, we are thus dealing with a subject that we usually have only been able to trace back to the sagas, where it’s part of the Völuspá - The Seeress’ Prophecy - which also comes in three different versions: Poetic Edda (1270), Hauksbók (later version, 1310) and Snorres Edda (approx. 1300-1350). While much aligns between the three, there are also differences between versions to add to the complexity. So if anyone wishes to delve deeper into the matter, be aware of these differences when referencing.
Now, time for another disclaimer: The typical saga focuses on a single story, and is relatively easy to follow. Völuspá is a different beast entirely in comparison since it is stacked with references to other tales and myths. Some of these references we still can't understand to this date, because the stories have - quite literally - been lost. So the text itself comes with an enormous expectation to its reader that one understands the references given, much of which is still being academically debated to this very day. This also shows how much has potentially been lost in time, so remember, as stated in the beginning: We can’t be sure of much, if anything.
However, the most likely key to both answers lies in the title: The Seeress' Prophecy. Ragnarok is considered the end of the world as it was known in three ways: Cultural, mythological and natural. Culturally it would mean the collapse of society and the norms that built it: Family feuds spin out of control, no one has anything to spare in regards to hospitality, and guests are turned away because the households don’t have enough for themselves. In modern terms: It’s a free for all. Mythologically Ragnarok is the death of the Gods and the Giants (Jötunn). Nobody wins, everybody dies.
In the natural aspect comes the notion of Fimbulwinter: Three winters in a row with no summers. The sun would be swallowed by a wolf and volcanos erupt. Fire scorches the earth. The flames of Surt will consume Asgard, Midgard, humans and gods. The World Tree of Yggdrasil will tremble, and the earth shall shake. Something truly fascinating is the aspect of Fimbulwinter in particular may have been a prophecy based on reality. A paper published in 1999 (Axboe) raised this very possibility due to an unusually cold period in 536-546, where archaeological findings discovered a very high concentration of gold being dug down. This has been a topic of great discussion among archaeologists, but according to a study from 2017 “it appears that the decline in the archaeological record cannot be explained as social changes such as consolidated wealth and power. The uniformity of the decline seems to point towards a decimated population brought forth by the onslaught of a colder climate and possibly plague to some degree” (Vethrus)
So it is entirely possible that the notion and idea of Fimbulwinter came from a real event that took place in Scandinavia. However, Ragnarok from the perspective of norse world view (according to sagas) was not something that had happened already, but something that would happen because it was destined to do so. Aspects of prophecy and destiny didn’t just align for the norse gods, but was considered to be part of every human’s life. It’s an entire topic in itself, but from what we can read in sagas, fate and destiny was integral part of the order of the world, spun/controlled by the Norns at the feet of the world tree Yggdrasil, but not in a sense that you as an individual had no control or free will. Fate in regards to the sagas was likely a matter of what happens, and that it WILL happen.
To summarize: Mythical aspects of Ragnarok, the Fimbulwinter, was likely born of a real disaster that previously occurred, however Ragnarok as an event was still considered to come. There is no inherent evidence of there being an understanding of the world as in some sort of timeloop, since the knowledge of the events of Ragnarok stemmed from prophecy and norse understanding of fate. Allegedly.
Edit: Further disclaimer