I'm trying to play a traditional battle mage - heavy armor, 1 handed weapon in my right hand, offensive spell in my left hand.

In my quick menu, I have #1 mapped to the mace, #2 mapped to the destruction spell and #3 mapped to a healing spell.

Without pausing the game, what keys do I have to click on so I can un-equip the spell in my left hand so I can free up my left hand for blocking an attacking with my weapon? I think the in-game term for weapon blocking is "Riposte."

If I click on the #2 key again, then it un-equips my weapon and puts the same destruction spell in both hands.

So far, I've only figured out two ways around this flaw in the UI and neither of them are really adequate:

1) click on Q to pause the game, and then manually right and left click on the weapon and spell so that it frees up my left hand. This is very tedious and immersion breaking. Because I only need to block with my weapon for a few seconds, it ends up taking me longer to un-equip the spell in my left hand than the window of time I need to be able to riposte. In other words, it takes me more than a couple of seconds to bring up the Q man, scroll through my really long list of items to my weapon and spell, and then manually right and left click the items I want in my right and left hands. I need to be able to get ready to riposte in one click; as it is, I end up dragging the fight on much longer than it has to be because I'm constantly pausing the game.
2) carry a shield and assign it to the #4 favorite key. Battle mages generally don't carry shields and I want to maximize my carrying capacity for the time being.

Why can't I define an item set on the PC version?

I should be able to play the game in the way I want to play it without structuring my equipment and abilities around the limitations of the UI.

I have this problem too

As a khajiit I have pretty solid hand to hand damage in the early game, but there seems to be no way to swap between spell and hand