1) What is an "inseparable preposition"??
When I was learning Hebrew I was told that it is single letters (such as lamedh you mention with regard to Jer 29:10) attached to words. I realise now I must have made a mistake in using this term for the Coptic here since an "inseparable" article is involved rather than a preposition.
2) You speak as if the "force" of the rendering is already known and I am "undermining" it by seeking illuminative (counter)-examples. If that is the case, when and where did you learn Coptic? Because the actual sense of the indefinite article is NOT known until one either understands the language or looks at how the article is used.
I am relying entirely on the comments from the scholar from the link provided by Narkissos who has the adavantage over all of us in that he is apparently familiar with Coptic. So it is not as if we are going at the subject blind. What we have is a scholar familiar with Coptic who has stated that the text presents the second "god" as indefinite; so that is why you can be said to be trying to "undermine" that position: not my position, his.
That is why I pointed out the text in Ephesians because it clearly uses the indefinite article in a way that CONTRASTS with English, is more generalized than English, and more critically for John 1:1, shows that the Father can be "a god" too. It suggests that some caution may be needed in interpreting the phrase, and the further examples from the Passio Mecurii may possibly suggest the same thing as well.
I am not convinced that Eph 4 does provide an illuminative parallel for the reason Narkissos has given. And you have not addressed his response as far as I can see.
But you need to bear in mind that translation is an interpretive act, it often tells more about the thinking of the translator (especially in making choices between different options of rendering an ambiguous text) than it does about the text being translated.
Be that as it may, it cannot be entirely uninteresting to note how translators dealt with the problematic construction, especially when they were so much closer to the composition of the original than later exegetes who have grappled with text.
To remedy somewhat the tone of my previous message I want to thank you Leolaia for you knowledgeable and informative comments on this subject. I am learning a lot from reading the contributions from Narkissos and yourself.
I approach the issue from the point of view that I am no longer convinced of the inspiration of any sacred texts. However, I have also come increasingly to the view that JWs can claim far greater support for their theological positions from the sacred texts and early Christian writings than their critics often claim. As BeDuhn argues, JWs may have allowed themselves a more reflexive reading of the sacred texts as a result of their more radical break from mainstream Christianity than that occasioned by the Reformation. That is not to say they have everything right (far from it, and they have powerful "traditions" of their own which warp their reading of many texts), but they did approach the Bible with a freshness thay allowed them to rediscover some aspects of the theology of the early church that centuries of dogmatic reflection had somewhat obscured.