I may be wrong since this is advanced mathematics but I think the Klein cube principle can actually be used for any R^X beyond R^4, similar to the Klein bottle that Felix Klein also invented, so that doesn't really seem to limit it to only 4D, feel free to ask someone more knowledgeable in dimensional math like Ultima or DT though.
Every source I see says they can only exist in the fourth. It’s also commonly known to be a fourth dimensional object.
And again, the moon cell core exists in 4D INS
Will say at least that something that probably supports it not being limited only to 4D is the fact that Kiara is described as
more complex than a 4D cube, which along with the 8D statement should indicate that the Moon Cell isn't really limited to only 4D.
It’s describing her mental structure,which is basically a big nothing burger. It could definitely just be similar to Tsukihime Kiara in which she can perceive a dimension above the 4th.
"There are many who have reached the status of a god as a human... but to think there could be one who surpasses a god while still human. No, seriously... what is that?! Her mental structure can't even be represented by a hyperdimensional cube!! It's like she’s got a universe inside her belly!"
「人の身で神に至ったものは数あれど……人の身のまま神を凌駕するものがいようとは。いや、そもそも……何ですかアレ! 精神構造が超次元立方体でも表せねー!! 腹ん中に宇宙作ってんじゃねーですよ!
Scaling-wise, it would also make sense if the Moon Cell wasn't limited to 4D only since it could apparently replicate Avalon (
the sheath is in Artoria room) and recently demonstrated the ability to just straight-up pierce through the anti-purge defense granted of Avalon.
Avalon is never noted to be anti-purge defense, or shows up in OC3. In fact, it’s noted to be of a level beyond defense.
Also, the sheath itself isn’t 6D, just the protection it grants, it’s pretty much just a regular object without the TNR.
Ignoring the fact that it weaponizes the time-lock, which is
1. A 4th dimensional phenomena that works by manipulating the Timelock system, something Avalon is seperate from
and
2. Pretty sure this is part of the things it’s explicitly said to block.
In regards to the Arcueid statement, I don't think it really contradicts stuff since we know that nothing from Earth can destroy Earth due to the defensive systems in place that prevent even things with the necessary strength from being able to do it, this is the entire reason of why Earth goes out of its way to fight aliens, as these defensive systems don't apply to them, hence why Excalibur had to be created in the
Inner Sea (Avalon) and universe's furnace to make it the strongest possible weapon, and from Lostbelt 7 we know that even a full power Excalibur used against an alien threat can't kill or significantly hurt ORT (and as result Earth was destroyed because of its strongest weapon being useless).
Arcueid says nothing in general,not just earth.
“defensive systems in place that prevent even things with the necessary strength from being able to do it,”
Which defensive systems? If you’re talking about the counter force, it explicitly does not act as a shield, it just summons guardians to get rid of threats, or influences human activity so that they get rid of the threats instead.
Even stuff like ORT is affected by the Earth’s texture, as it is affected by parallel world differences.
It’s also explained in the OP why destroying the “earth” in a singular timeline being equal to destroying Avalon is illogical.
And maybe it's just me but I don't see how the Tsukihime Kiara statement (the one about she willingly not ascending to a higher plane) means that the world caps at 4D, it says the oppose actually, that there are things beyond the 3D material plane of mortals (as in the human texture where normal people live) and that she is utter shit compared to those above so she prefers to stay among the 3D mortals to feel better with herself.
Except Kiara and Roa note that these higher dimensions are “another world”, and is the “outside”, and that she willingly returned to “this one”