I leave for a few hours and now we're discussing 3-B touhou? What the hell?
(Most of this is directed at Promestein but it might apply to other people's arguments, who knows)
Apologies if some of these topics have been previously addressed, I missed a lot and it's hard to take in everything all at once.
Okay, starting off, I'm gonna mention that there are still a handful of universal feats that could be used. The Occult Balls are stated to have infinite energy flowing out of them (Sagume created the one in question so she should scale), Youmu's spell card that we used to argue for infinite speed covers an infinite area using power drawn from the moon, and Eirin can use Klein Bottles, which contain infinite mass and are 4D (she uses this as an attack via one of her spell cards, so it should be acceptable for AP).
I have a big issue with implying that Akyuu was just referring to former hell in that article, because PMiSS came out 2 years before SA. Why would she be referring to a concept that literally did not exist at the time of writing? To put it another way, why would ZUN have a character reference a location he hadn't even designed yet? It's a massive leap in logic to assume ZUN was talking about former hell in that instance, and the authorial intent was clearly to have Akyuu refer to hell as a broad concept, not just one specific location that didn't even exist yet. So in my opinion, throwing away one of the only decisive statements regarding the size of heaven and the netherworld using impossible logic like this is completely ridiculous.
Also, Akyuu's inaccuracies are either due to her misinterpreting actual canon information (thinking Kaguya and Eirin are humans, thinking Mokou came from a clan of youkai hunting ninjas) or being deliberately fed false information (youkai abilities are self reported, so it's easy to assume some of them were lying to make themselves look good, the least debatable example of which is Yukari's 'undermining all of reality' statement), neither of which have evidence to apply to her statement on hell and the Netherworld. And if you want to argue she was just speculating, she
also speculates regarding Sakuya's origins, but in that article she clearly states she is speculating. If she were doing the same in the article where she compares hell and the Netherworld, she would say so. The statement itself being vague doesn't really disprove anything, either. It's the only definitive statement we have regarding the size of two extremely important Otherworlds, so unless we have explicit evidence that it is incorrect, it's the best we have to go off of. We have to work with what we're given, and touhou never gives that much.
Last thing regarding this point. ZUN has outright stated that there are no borders between hells, and trying to separate them is essentially pointless. Make of that what you will.
As for Miko absorbing Kaguya's infinite corridor being an outlier, keep in mind that it was stated to be a 'hermit art', which implies other hermits should be capable of replicating it (Seiga, Kasen, and Futo). Also, something being 'weird' or 'hardly explained' does not immediately disqualify a feat. What else would even need to be explained about the feat, anyways? She's clearly absorbing infinite space. There's not an implication or anything, it's right there in the text.
I also wanna ask if we're absolutely sure that Suika destroying the sky isn't destroying Heaven, as these statements from PMiSS and AoCF would imply. To me it sounds like shattering the sky is equivalent to shattering Heaven.
There's also Dream World stuff, but someone is making their own post about that so I'll hold off.
So in my opinion, using the context and evidence that you've given, it still seems like there are a handful of feats left to discuss, Akyuu's statement regarding hell is
not referring to former hell, so Heaven and the Netherworld are still infinite via that statement as a result, so Tenshi shaking Heaven is H3-A, Miko's corridor absorbing feat is still valid (and it should scale to 3 other characters, who have their own scaling chains), Suika destroying Heaven is possibly still valid, Having a few notable feats debunked hurts, but as it stands now I don't think it's enough to downgrade the verse below H3-A. And given that 4 separate characters would have an L2-C feat (remember that you cannot affect space without affecting time in touhou), I think a flat L2-C rating is much more appropriate. I'd obviously like to keep the 2-C ratings, but that's reliant on Dream World Stuff, which I'm leaving to someone far more knowledgeable on that topic.