http://www.imperial-library.info/content/cosmology
The first one on planets says they appear as planets due to mortal mental stress. I guess we do know where they are but they have an infinite mass of infinite size. Sounds like a black hole or something mathematically similar. Black holes have infinite gravity.
I think of it this way; everything the T0 has suggests that the planes are not spherical, but flat. Even if the Imperials know that there are other continents, I've never heard of them sailing around the world. To them, the idea of the planets and planes being spherical must sound crazy.
What powers did Sanguine and Namira grant to mortals? You're not referring to the usual artifacts, are you? Those are just custom objects with the usual stock spells/effects added to them if you really think about it.
But you're saying that in-lore, Daedra/Aedra have magical powers that are not accessible by mortals? There is no spell to make dogs made of fire fall from the sky but Shegorath has the ability to give that power to a mortal mage if he so chose?
Sanguine gave you the power to make the target's clothes disappear (as well as the caster's) but it only worked at a certain place at a certain time on certain people. Namira gave the power to extinguish torches; not nearly as impressive, but still unique. Molag Bal gave you the ability to survive what would otherwise kill you.
And yes, I believe Daedra and Aedra can do things that mortals cannot. It could be that they're of a higher subgradient, or simply that the possess far greater knowledge than most mortals. As I recall, Mankar Camoran, with his given resources, did some pretty amazing things for a mortal.
On a side note, in my fanfic, my character Vedaa was created by the 16 daedra and two of the aedra (Dibella & Mara). With that, would it be lore-appropriate if she were to have unusual and uncanny magical abilities that no other mortals would have? Like using magic to tinker with thermodynamics and physics?
We've got the Dragonborn and those born under birthsigns with special powers, so yeah, they could have special powers.
It says in http://www.imperial-library.info/content/morrowind-varieties-faith-empire that in Cyrodiilic legend, Magus has the ability to lend powerful magicians some of his power. I kind of wonder if Gyron Vardengroet from the book http://www.imperial-library.info/content/daggerfall-sage was being lent some of the powers from Magus. Couldn't any god or daedric prince lend their powers to a mortal if they wanted to?
I don't know what the limits are as to what powers can be lent to mortals. If there are limits. But yes, Oblivion has had both Daedra and Divines give or lend powers to mortals.
I think of mortal mental stress as being similar to us trying to visualize what a 4-D hypercube, tesseract, looks like, where it is a cube within a cube. In 3-D projection, the inside cube is connected at 45 degree angels to the outside cube. But a real 4D cube has all right angles(an extra right angle to the x,y,z right angles), which we cannot comprehend because we are trapped in 3 dimensions.
Either that or, as I said at the top of my post, attempting to comprehend the planes as spherical when everything you've experienced suggests they're actually flat.