"～ことを" itself is not that idiomatic, but I think "～があらんことを" is idiomatic.
神のご加護があらんことを: This sounds natural to me. You can safely say "Xがあらんことを" is an archaic-sounding idiomatic phrase which means "I wish you X" or "May there be X". This is a fixed pattern used mainly by priests, and I have never wondered what is omitted after it. I think those who don't go to church regularly would encounter this expression only in fiction, and understand it without thinking its grammar at all.
神のご加護があろうことを: Maybe not too bad, but I'm not familiar with this. A Google search for "加護があろうことを" gave only one result. Of course "Xがあろうこと" can simply mean "Xがあるであろうこと" or "that X probably exists" in modern Japanese, and you can find thousands of such examples on the net. It's just uncommon as a wish.
神のご加護があることを or 神のご加護がありますことを: Not common nor idiomatic, but understandable as the plain sentence where something like "願っています" is omitted at the end. "神のご加護があることを願っています" is a perfectly grammatical sentence which only uses the simplest contemporary grammar. So it sounds businesslike and matter-of-fact as compared with "ご加護があらんことを", which has a religious atmosphere.
EDIT: Note that 「～があらんことを」 is a grandiose phrase. Religious characters in manga and games say this all the time, but I don't think ordinary Japanese Christians use this often in daily conversations today. 「～がありますように」 is much more common both among secular people and religious people.