Undoubtedly we know very little for certain about Norse mythology and rely a lot on textual evidence only, but that could also be said of much of history. As this comment by glashgkullthethird mentions, there actually are many attestations (or at least clues) of historical pagan practices and beliefs. I've noticed that lately there've been many threads on this subreddit and in others, in which people question the reliability of our sources on Norse religion, and while that brings many healthy discussions (especially considering the popularity of media portraying norse society and religion, as if we had a perfectly clear understanding of them), we should also be careful not to fall into the other extreme, thinking we don't know anything at all with any certainty whatsoever. A middle-ground approach would be better, acknowledging we have numerous gaps in our understanding of norse society and may never know with 100% certainty what their beliefs were, but also recognizing that those limitations don't keep us from making educated guesses with our knowledge of history in general. In my particular field, reconstructed languages, the same problem arises, where by lack of hard evidence people just assume it's all uncertain, not knowing how much theory and analysis our "guesses" entail. It's kind of like how astrophysicists have never set foot on a planet in the other side of the galaxy or ever sent a robot there, but with mathematics and physics they can make very well educated guesses on the planet's atmospheric composition, it's density, size, etc.

With that in mind, let's talk about the Ragnarok. Our main source on the nordic belief that in the end of times there was going to be a battle between gods and giants which would result in the birth of a new world is the Völuspá, a poem recorded in the Poetic Edda. While the Prose Edda was written by Snorri Sturluson, a Christian Icelandic historian, we don't know the Poetic Edda's author. It is a compliation of poems whose time of writing varies greatly, as many historians still argue about their precise dating, with some estimates going as far back as the 9th century. This Edda is preserved mostly in the Codex Regius, an icelandic manuscript written in the 13th century, well after the country's conversion in the 11th century. And, with the poems being written probably much before, you could raise into question how well preserved from pagan times they were and how we can know that they're not completely fake. However, by studying how christian authors of the time wrote, how they wrote about heathens and foreigners and how they wrote about the past, we can actually analyse the christian influences in the text with some degree of accuracy. We know that, instead of falsifying stories and myths in the way you suggest with the Ragnarok, Christian authors used to either euhemerize pagan deities, as the Prose Edda's prologue shows, or to outright treat them like demons, as in the Old Saxon Baptismal Vow. In the Völuspá, the points most frequently raised as being christian influences are two: the mention of an "Almighty who rules over all", and since that is a somewhat foreign concept to norse belief (as far as we can tell) when not in relation to Odin (the Alfǫðr, which may mean either "All-father" or "The one who puts things in order"), we can infer that as a Christian insertion; and the composition of the poem as being told by a prophetess, which some suggest as deriving from the Sibylline Oracles, but we have many attestations of prophetesses in nordic and continental germanic pagan societies and their role as oracles telling the future, as in Tacitus's Germania, Strabo's Geographica, Eiríks saga rauða and Gull-Þóris saga, so that is a little less certain.

Besides those aspects, there's not much in the Völuspá which suggests christian influence. We could, of course, believe that everything written there about the Ragnarok is indeed part of a huge conspiracy by christians to convert pagans, but there is no evidence of that. What we do have evidence of is the practice of skaldic poetry in Icelandic medieval society, poets who sang poems about the deeds of great kings and heroes and mythologycal stories, even after the conversion to christianity. Even in Snorri's Edda we can read the Skáldskaparmál and the Háttatal, plainly showing the author's artistic intentions and well separated from the obviously euhemerized christian prologue. So, instead of the Ragnarok being a fake story made up by Christians throughout the centuries (who also managed to destroy any texts talking about this conspiracy and never again or before used this practice to convert heathens), it is much more probable that it is actually a myth believed in by pagans in historical times which poets would sing at king's and noble's courts, sometimes changing a verse here or there, reflecting their new beliefs.

This post by /u/Steelcan909 goes into the source of Viking religious beliefs and answers some of your questions.