Before starting I want to say that I do agree with a few things, but I disagree with some other. I’m also a semi-anime only, that’s to say I’ve seen the entire anime and read the manga up to the Swordsmith Village Arc, so I can’t reply to your whole post. By the way, English isn’t my native language, so I apologize in advance for the mistakes I might make.
Debunking the Killer Fish Calc
We have seen that Breathing Styles allow for AoE attacks that could easily cut them all at once,
shown here with Muichiro himself hitting all 10.000 fish with one slash.
I also think that the 10,000 fish calc isn’t MHS, but I disagree with Muichiro being able to cut all the fishes by swinging his sword once. The panel you provide does not shows Muichiro slicing the fishes, he just tossed them away with air pressure, and as far as I know he has never been shown to be capable of cutting stuff with something like a shockwave, in fact, the only slayer I know who has shown been capable of that is Sanemi.
And Breathing Style Techniques never consist of spamming a massive number of slashes
1.- Even if no other slayer has been shown to spam attacks with his breathing style, that does not necessarily means that Muichiro didn’t spammed slashes to cut Gyokko’s fishes.
2.-
Sound breathing Fifth and Fourth form actually consist of spamming slashes, and Love breathing third form is similar, as the same panel you shared shows Mitsuri “cutting“ various of Zohakuten’s lightnings.
here is Mitsuri's Breathing Style covering approximately 100x the necessary volume to completely vaporize that attack.
Mitsuri’s sword is evidently way longer than Muichiro’s, plus in the anime her swords seem to be capable of extending.
Debunking that the Fish must have been at least the speed of sound
You can actually tell that Gyokko’s attack was serious as he got
(really) pissed off when Muichiro told him his pots were deformed, in fact, the 10,000 fishes was Gyokko‘s last resort before transforming. By the way,
Zenitsu has also been shown to move at supersonic speeds since his very debut, and Gyokko scales way above him and Kyogai, even if his attack wasn’t completely serious it is pretty safe to assume sonic speeds for the fishes.
I want to add that I'm not defending that Muichiro sliced the fishes ony by one, nor that he performed 5000ish swings, but slicing all the fishes by swinging his sword once sounds as improbable as slicing them one by one. I actually calculated this feat again
here and managed to get the maximum number of fishes that Muichiro could have cut with every slice of his sword, although it hasn't been revised yet.
Debunking the Six-Fold Thunder Flash calc
I agree with the calc for Zenitsu not being valid as the spider brother was never shown to have such a low reaction time.
Zenitsu's speed scaling across the series seems arbitrary
Mostly agree as God Speed actually increases Zenitsu’s speed (although by an unknown amount like you said).
Massive calc stacking + Many, if not all, speed calcs are based off the Muichiro calc
Mostly agree as there is a calculation that shows
base Mitsuri moving at low hypersonic speeds (I know this isn’t even close to MHS, but I’m just showing that not all the verse’s speed is based on the fishes calc).
No evidence for Kaigaku being comparable to previous UM6
I’ll skip this since I haven’t reached that part of the manga yet.
Even if Kaigaku is as strong as previous UM6, he is still not Massively Hypersonic
I’m going to skip most of the discussion here, but I’ll refer to this one part:
all the other Pillars are far superior to UM6. This is proven by Tanjiro, who could fight Gyutaro and deflect his attacks before he was Marked mid-fight
If I understand correctly, you are saying that Tanjiro was somewhat comparable to Gyutaro, which is simply not true. Tanjiro was way inferior to Gyutaro, in their first confrontation
Tanjiro got instantly blitzed and only survived because Tengen saved him, in fact, Tengen saved him at least thrice during the whole battle. Tanjiro was indeed capable of deflecting Gyutaro’s attacks, yet not because of his strength, but because
Water Breathing is more effective as a defensive style, in contrast, Tengen blocked and parried Gyutaro’s attacks by simply responding with more attacks. Tanjiro’s strength was nowhere near Gyutaro’s, as Tanjiro himself states, his katana would have broken if he tried to stop Gyutaro’s attacks through strength alone. Also, Tanjiro and Gyutaro barely clashed during the battle, Tanjiro was more of a support, the only part were they actually clash, was at the end of the fight after Tanjiro failed to cut his head, and Tanjiro could barely keep up with a Gyutaro who was still poisoned, he almost got killed actually (again), but Tengen interrupted and saved him (again). So no, Gyutaro is far stronger and faster than Tanjiro.
Therefore you cannot compare Zenitsu's attacks to actual Lightning
Agree, although I’ve never seen someone actually saying that Zenitsu is as fast as lightning just because of these metaphors or Lightning breathing.
Another false Speed of Lightning feat that is often brought up of is
Mitsuri cutting Lightning. Now apart from the fact that real Lightning is immaterial and cannot be cut (and as we established, Demon Slayers aren't magical sorcerers so they don't have any superpowers like Haki to hit Lightning), we cannot assume that conjured Lightning has the traits of real Lightning.
Actually, real lightning is material, it is something like a huge but really short flow of electrons (and negatively charged particles) through air, something like a stream of water. The reason you can’t cut it is similar to the reason you can’t cut the water coming from a hose, plus they’re way too fast for anyone to try and cut them, and if you get too close to them, they’d probably go through your body and get you killed. So, you can sort of “cut” lightning, but that would most likely briefly interrupt the flow of electrons, like when you try to cut the water coming out of a tap, this pretty much explains why Zohakuten’s lightnings were still there after Mitsuri cut them. By the way, lightning summoned by Hantengu actually showed properties of real lightning, like being able to electrocute Tanjiro, Nezuko and Genya; and not being capable to flow through non-conductive materials like Urogi’s foot.
The Blood Demon Art in question was
a combined attack of Thunder and Sound, and the panel clearly depicts that both the Sound waves and the Lightning strikes had roughly the same attack speed
There is no way to prove that from a single static image, in addition, the lightning bolts were clearly at ground level while the sonic wave still hadn’t reached Mitsuri.
Lightning moving more than 300.000x faster than Sound.
This is also wrong, the
average speed for lighting is 440,000 m/s, while the speed of sound is 343 m/s, so lightning is 1283 times faster than sound, even the top speed for lightning is far from being 300,000 times faster than sound.
So either we have two massive outliers in a row of Mitsuri cutting Lightning effortlessly, only to immediately get hit by a Sound attack that is more than 300.000x slower than the Lightning attack she just countered
This is no anti-feat, Mitsuri got caught into Zohakuten’s sonic wave because it was an AOE attack and she was in mid air, so there was no way she could have dodged and getting covered would have been useless, yet she actually defended herself from the attack by tensing her muscles before getting hit, this was
explicitly stated by Zohakuten plus we can see Mitsuri keeping his stance even though she was unconscious, which is a consequence of her tensed muscles.
Gyutaro being hit by a Kunai from one of Tengen's wives
I think it’s pretty clear that Gyutaro just got caught out of guard, Tengen had just cut his legs and he was still staring at him right before getting hit by the kunai. This happens all the time in anime/manga/comics/movies/etc., so it’s nothing to worry about.
Muzan not being able to dodge a point-blank explosion
I haven’t reached that part of the manga yet, so I can’t reply.
Genyu used a conventional shotgun against Upper Moon 4 and successfully landed multiple hits.
That is not a conventional shotgun as Tanjiro stated it smelled like a nichirin blade. One of the panels you just provided clearly shows how Sekido saw the bullets before they hit him and you also skipped the part where
Tanjiro tells Genya that Hantengu’s clones were letting him behead them on purpose, you can even see Karaku smiling after getting his head cut off. Aizetsu let Genya hit him since he had already pierced him and was expecting Genya to die in the next few moments, plus they didn’t really care about getting shot since they could regenerate immediately, just like Aizetsu did. There is even a panel were
Sekido blocks one of Genya’s shots, which makes sense as Sekido was being more serious than the other emotions, who were pretty much toying with Tanjiro and company.
I didn't read all of the replies before writing this, so I might be repeating a few things.