Alright. I've taken a look through all of the discussion and support/rebuttals since I left (I've been quite sick recently, for those of you who didn't know, so I haven't been in a position to do in depth debating). I think some genuinely good points have been brought up on both sides of the argument, and for the opposing side Kepekley in particular has made some pretty good points. I'm going to try and quickly summarise my thoughts on the arguments thus far. Also, I apologise if I tangle my words a bit while writing. I'm still a bit sick, just well enough that I'm willing to further debate the topic.
Honestly, I've reached a relatively neutral stance on the 2-C debate. Quite simply, arguments revolving around space-time are outside my field of expertise, and it's clear I misunderstood quite a lot about timelines and separation. I'll do further research into the distinctions between separate space-times before officially agreeing/disagreeing with the 2-C tiering, mainly because I find the evidence on both sides to be somewhat inconclusive. Until then, consider me neutral.
However, I'm still in agreement with the High 3-A tiering in comparision. The definition of countable infinity is, to slightly paraphrase, an unending set. In the context of an "infinite sized universe", it would mean that the "set" of it's size is unending. As such, I don't see any reason to consider the various and consistent statements regarding the demon world as endless/unending to be invalid. Not only do we have the statements I mentioned in the main post, but we also have the extra statement from within the Sargasso's description about the demon world being unending.
To put it bluntly, I still see no problem with the demon world being considered infinite in size. There are almost no occassions in the entire series where a proper scale for the demon world has been shown, and in the cases I've provided they all consistently back eachother up in confirming the infinite size of the demon world. On top of that, as far as I've been able to find (since I've gone very in-depth attempting to find the scale of the demon world) there is not a single statement about the size of the demon world in the entire series that contradicts the statements I have shown. The demon world's size has barely been mentioned in the entire series, and every time it has been mentioned the given statements strongly imply the infinite size of the demon world. If it were finite, all of the statements about (for example) the "endless sea of the underworld" would be completely illogical and inconsistent.
To summarise, all in all I'm still fully confident in the High 3-A tiering I have suggested for feats related to the demon world. If there is still major doubt about it because of, say, the infrequency of the statements then I'm willing to compromise for something along the lines of "Likely High 3-A" instead.
When it comes to the Saviour, I'll admit, Follow Doctor Freeman makes a good point. It is a quite strange part of the game that could be assumed to be outside of the given narrative (especially in a series with a strong sense of ludonarrative dissonance like Devil May Cry). However, I must confess that I don't find this to be reasonable justification to say that it did not happen. You do still play through it, and we never assume other parts of the game to have simply never existed. The only reasons I can see to say that it did not happen are the fact that it is never shown in cutscenes (which honestly doesn't prove much, since there are no cutscenes during the sequence anyway) and the fact that it carries a strange, ethereal feel. Other than that though, the walls of text mentioned throughout may be in weird rhymes but they are still informative enough that passing it off as simply never happening would be outright odd.
In the end, I suppose I can see why considering this sequence to be real was a bit presumptuous of me, but at the same time I still don't find it particularly reasonable to consider it outright false based purely on obscure, almost irrelevant factors. I think a rating of either "Likely/Possibly 4-A" seems far more reasonable to me in hindsight. I'd like some input on this point.
I want to be clear about the Nightmare section that, while I have depicted the supporting opinion on it for the sake of insiting debate into a long-standing question, I am, and have always been unquestionably neutral on it. I still am now, so I really don't have much to say about it.
With the Bolverk section, I still believe that he should recieve a downgrade. His "Possibly 3-A" rating is based on having survived a battle with Sparda in the past, and having fought a casual DMC2 Dante. Problem is, neither of these are particularly applicable feats. If we consider having fought Sparda to be a Possibly 3-A feat, then that would scale to characters like Griffon and, as mentioned in the post, Beowulf. Fighting a casual Dante has never been an applicable feat either, as the same line of logic could be used for the sake of upgrading characters like Echidna, who fought a casual DMC4 Dante. You could argue that having done both of them accounts for more, but two wrongs don't make a right. Having two equally invalid feats to justify "Possibly 3-A" does not make either of them valid. As I mentioned, there were multiple ways to approach the situation, and I'm still quite confident that removing Bolverk's "Possibly 3-A" tier is the best approach.
And I think that about covers it! I apologise again if I tripped over on my words while writing this. I'm currently still sick and writing this late in the afternoon while painfully exhausted, so I'm probably not in any good position to be writing Hemingway novels to say the least. I hope my writing is at least legible.