The mana value of a split card not on the stack or a fused split spell on the stack is determined by the combined mana cost of its two halves
On the other hand, proxies are substitutes for expensive cards you wont risk putting into your deck, perhaps for a Cube or EDH deck you want to build, or for aesthetic reasons
I've also seen a few mentions of Vivi Ornitier, but we'll have to see if it's fast enough to keep up with the current meta
Excellent design on this blue commander, both flavorfully and mechanically
Let's go back to the example where your opponent is attacking with a lethal board of creaturesyou can flash in Angel of Grace to stay alive, block in a way that puts Angel of Grace in your graveyard, and then spend your next turn going back up to 10 life, hopefully enough to survive another turn as well (giving you time to draw another Angel of Grace!)
However, it is different in two major ways