Now ask yourself if that was deliberate because it's how Bethesda wanted the lore to be, or if it was compromise for mechanical reasons. In other words, lore-wise, Player can leap over a wall of the city and continue on with his journey. Game mechanics wise, Player had to go through city gates in TESIV because cities were interior cells. Some part of me says that Bethesda wanted those specific spells to only be available in Shivering Isles because it would cause game mechanic problems if they allowed Player to cast them any where any time.
A very large part of TES IV revolved around venturing into Oblivion. So much that the game wasn't named TES IV: Cyrodiil. This continued into the expansion. I think that if they intended for things to work differently in the realms of Oblivion they would have made it that way. Yes, you can use the paintbrush glitch to get over the wall, but that was a glitch; they didn't mean for people to be able to get over the walls.
Just to make sure we're on the right page, you're referring to these 24 spell IDs? http://www.uesp.net/wiki/Shivering:Magic
I can see how Manipulate Weather would only be relevant to Shivering Isles. But I don't see why "Legs of Fortitude" or any of the summoning spells wouldn't work in the Mundus, lore-wise.
I'm referring to three spells that cannot be cast everywhere; Manipulate Weather, Sheogorath's Protection, and Summon Haskil; I believe these are the only spells and powers that the PC can use that utilize the magic of the Isles. And let's not forget the Staff of Sheogorath, which can be instantly recharged at where all of these magics come from.
When Haskill says these magics are rooted in the Isles, it sounds kinda like the Levitation Act excuse for not being able to render levitation in the game software. Bethesda couldn't implement levitation and so they created that levitation act to "fill in the hole" of the lore. I'm thinking the same thing was done with the SI expansion pack. But what's said is done. And in a way, it does add more mystery. What other unique magical abilities/spells could there be in other Oblivion realms and/or pocket realms. My question is why does it work like that? When you look at the sun(Magnus) in Shivering Isles, are you looking at the same sun that mortals see from Nirn?
In the vanilla main quest, Martin notes that the opposite (or simply Daedric counterpart) of a Great Welkynd Stone, is a Great Sigil Stone. I believe Sigil Stones have Daedric magic trapped within, enabling them to be taken anywhere and make enchantments that work anywhere. Still, we must remember that the Princes ultimately command their realm's power, and it seems they can bring it to Nirn. Sheogorath was able to change the sky and make flaming dogs fall from the sky, and others, like Sanguine and Namira, were able to grant very unique powers to mortals. Perhaps it requires great power to bring Daedric power to Nirn, or the Princes simply keep a tight enough control on their realm's magics that it's rarely seen outside their realm.
As for Magnus, I do have a theory about the placement of Oblivion.
I theorize that the planes of Oblivion lie out in the void of Oblivion, out there in the sky, like extrasolar planets, and that while Sheogorath's realm is near a star similar to Magnus, Mehrunes Dagon's realm is near a dead star (considering it's the Deadlands, this stands to reason). Not even the Dwemer, with their telescopes, would have been able to see Oblivion planes floating around in the void. This would explain how the realms of Oblivion could exist out there among the void of Oblivion that one can see from Nirn. As for Aetherius, I don't know; it's certainly not accessible either by the Sunbirds or by the same methods used to access planes of Oblivion (explaining why the Mananauts failed to reach Aetherius but seem much more successful in Oblivion-related matters).
Yes, this does require a slightly different interpretation of the Cosmology text. I believe that the infinite masses and infinite distances described are not truly infinite, but so vast that non-portal travel is impossible (or at least implausible). This in turn would open the door for the idea that the stars aren't equidistant, but variably so far away it doesn't matter. And that perhaps the Temple-Zero Society isn't all knowing and infallible. They certainly http://www.uesp.net/wiki/Oblivion:Ancient_Tales_of_the_Dwemer#Part_II:_The_Seed in-universe. Perhaps the "mortal mental stress" is not defining what we visually see, but how we interpret and explain what we are seeing.