~あらんことを: Slight Variations and Idiomatic Degree

Question

Consider 神のご加護があらんことを

1. How is あらんことを different from あろうことを ?
   (c.f. archaic -an conjugation)
   Apart from sounding archaic, are there other functional differences?

2. How is {あらん・あろう}ことを different from あることを ?
   What does the volitional form do that the plain form does not in this phrase?

Is there a preference for one of the forms? I.e. is there an idiomatically frozen choice that is chosen over the others.

3. How idiomatic is ～ことを ? Is it such that we do not even consider if there is any possible verb that follows after it? If there is an implicit verb, would it be something like お願いします or 祈ります ?

Answer 1

"～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.

Answer 2

This answers the second question "What does the volitional form do that the plain form does not in this phrase?" :

Translation of one of the answers found in 知恵袋:

「あらんことを」は祈願文で、“御加護がある”ことは実現してほしいのですが、そうなるかならないかはまだ決まっていません。 それで「あることを」と言わないのです>

By saying あらんことを, one wishes to realise the state of 御加護がある. (i.e. to wish that the state transitions from 御加護がない to 御加護がある).

The future state of events cannot be said to be certain, and since we cannot guarantee the future state of 御加護がある, the phrase あることを is not used.

The volitional form allows the subjunctive mood which the plain form does not.