Many thanks, billberg23, this is very helpful!
There are still two or three points that need to be cleared up, hence three further questions:
1. I also think that Coleridge was alluding to Exodus 3:14, as I wrote in my initial post. However, is the perispomene over ὦν perfectly acceptable, or is it a slip for what should have been been ὤν, and would this affect the meaning? (I'm inclined to think that because the early LXX texts had no accents, ὁ ὤν and ὁ ὦν would have the same meaning, differing only in pronunciation, is that right?)
2. I think that ὦν can be well translated as ‘the one being’, ‘the being one’, or ‘the one who is’, but I would say that the best literal translation would be ‘he who is’, because ὁ is masculine, or as it is preferable––while being literal, that is––to retain the article, and so write: ‘the [male] one who is’, but then, ‘the [male] one’ is cumbersome, and a better English rendering of that meaning would be ‘he’, hence again, ‘he who is’. How does that sound?
3. I also think that ὦν cannot mean ‘the being’ in the sense of simply following an English-language word––the second item in this list, ‘Truth, Being, ὁ ὦν’––with its foreign-language equivalent (i.e. providing an appositive). Am I right to think this, or is it merely an opinion?
To explain my view, I think that writing ὁ ὦν after ‘Truth’ and ‘Being’ is not to end with an appositive in another language, but is rather to ascend as the list moves along, with the next item including the earlier one(s), from Truth, to Being, to God, which latter alone can say, as Coleridge put it elsewhere: ‘I am. Causa Sua. My own [ac]t is the Ground of my own existence’ (Marginalia III: 1065).