Thanks for bringing in another point of view. I probably am just going to make the water more muddy, unfortunately, but, I'll try.
Grace is God himself. We use the term energy to express the distinction between God's inner life (essence) and his life outside himself in relation to the world (energy).
Grace is God acting on us; to say that Anglicans have it right because they leave it "open" and thus "a mystery" to me doesn't sound better--I like the fact that starting with St. Basil, continuing with St. Gregory of Cyprus, and ending with St. Gregory Palamas, it is clear that grace is God himself--his presence, his action, his will, his love. Anything else, and it is extrinsic to him--whether it is a mystery or not.
The key is not whether it is defined but whether it is created or not. We say it is uncreated because it is always God and always a part of God. Grace is a synonym for God's love and action, his presence in the world.
I like your Anglican hymn you posted is very nice, btw.
This is going to get very philosophical for a moment, but just hang on.
The Episcopal Catechism does eventually get around to defining grace as "God's favor towards us, unearned and undeserved; by grace God forgives our sins, enlightens our minds, stirs our hearts, and strengthens our wills." It is the first clause that is critical, especially the words "unearned" and "undeserved".
I have looked briefly through the Catholic Encyclopedia. It is very hard to characterize their position against any other, because about five sentences after defining the word they dive into an Aristotlean philosophical jibber-jabber, not to mention spending an inordinate space in attacking Martin Luther. They make a distinction between "actual" and "sanctifying" grace, the former being acts of transient influence and the latter being a permanent state.
I can't find the issue of creation per se in any of this. Depending on how pedantic one wishes to be, one could argue either way on whether the action of God (what you are calling "energies") are part of His nature or something He creates. What I'm finding is that the question doesn't seem to interest the West. The big issues seem to be Pelagianism on the one hand, and (mis-)characterizing the Protestants on the other.
The Catholic Encyclopedia does not mention "theosis" in the Orthodox sense (they use the word in passing in an article on mysticism, but it's clear that the sense is different). Again, the impression is that, until recently, the Orthodox idea was not discussed, for good or ill. The issues of justification and superogation have tended to crowd everything else out.
From an Anglican perspective, the distinction between God's energies and his creation seems a bit on the philosophical side. Scripture is sure grace, yet it is of creation. A prayer for healing or for favorable weather must in the end be answered in creation. Anglicans of late have found the notion of theosis congenial, but it seems to me that now we are seeing grace as a sort of secret ingredient.
That comes back to the issue of grace as being like a substance. People talking about the sacraments are prone to a lot of talk that gets transactional. You do the sacramental act the right way, and you get a dose of Grace(tm). Pretty soon churches get into the business of being Sole Proprietors of Grace(tm) and one gets into a lot of niggling talk of exactly what it takes to do the magic act Right.