Very visually nice, but I'd suggest three changes:
1. Group equipped items together at the top of the item list.
2. Use the big 3D view for a character view instead, and either have the item view as a smaller frame within this area or provide a way to switch between character and item view (which would be optimal to me).
3. Smaller font.
With those three things added I'd personally say this is the best mockup I can imagine. Great work!
(Now start modding it in!

)
EDIT: also, the values for the selected item are shown twice (in list and in 3D view), which is a bit redundant. Perhaps only show item type and detailed enchantment info in 3D view.
EDIT2: oh, and show magicka and stamina under the health bar.
EDIT3 (I keep coming up with stuff after I've posted): Show the player's level somewhere.
1 and 3 should be possible; I'm not certain that 2 is. We'll know more about that as we get better at modding. Your Edits all look possible and perfectly fine to me, too.
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In regards to the UI the OP posted:
Looks rather nice! You'd use left and right arrow keys to switch between tabs (like different item or spell types), but I'm not sure how you'd easily tab between the Inventory, Spells, Map, and Skills UIs. You could obviously do it with the mouse, but I like the current UI's use of the keyboard to quickly tab through things. With a controller it'd obviously be the trigger buttons. For the keyboard, I suppose you'd have to close the UI, then open it back up to the proper location, or just use the mouse.
In general, though - very good ideas. Looks easy to use and much more useful than the current UI, while still retaining most of its utility. One addition I'd like is a 'Favorites' menu, though - probably the tab in the middle of the upper bar (the Skyrim logo), which would have details on all of the various Favorites that you've got assigned. Would allow you to customize them further, and see them all in a single place easier. Though making the middle button open the game's options screen would also be a good idea. *shrug*