Most ideas I toss here were just some quick thought to maximize the gameplay from my own analysis. I didn't put more than that. Some idea are indeed not well developed
Unblockable should be a designated move that is chargeable, maybe with 3 step (normal, guard shattering + hi knockback but no damage, hi damage little knockback but huge cool down).
So if you charge it a little, it does a lot of knockback, but if you charge it a lot, it does little knockback? That doesn't seem too intuitive... especially considering that many unblockable attacks are grabs.
In my mind the character spin on himself after fully charge by flipping his hit animation quickly as a visual feedback. But yeah I didn't think that much: I was influence by that http://critical-gaming.squarespace.com/blog/2010/9/16/neorpg.html
What if the player get a small invincible (but cannot attack either) time while he double tap down and go through the ground. It would make for a situational dodge and may lead to some sort of gameplay chokepoint.
I don't really like the idea of making the ability to dodge dependent on standing on a platform.
Like the idea above It was an attempt to shoehorn a gameplay influence by some game (edge garding in smash bros) ...
Regarding your idea, they seems fine, in the case of items summoning, i think guarding may act as a time cost, summoning isn't much a counter but an opportunities that guarding made safer.
At the end, all idea need playtest, it's only then we know they are good or bad.