Welcome to the board, Syntaxo....
However, in the Coptic translation of John 1:1c, those are the only Coptic options: either "the Word was a god/divine being" or "the Word was divine." The Coptic indefinite construction cannot support the definite English translation: "the Word was God."
I agree, from what has been posted in this thread about Coptic grammar, the value of the Coptic is that it is a witness against a definite reading of theos in John 1:1c (setting aside the possibility of theological Tendenz). But as you also bring out, there are two valid Coptic readings of u.noute -- a qualitative interpretation and an indefinite (i.e. class membership in a set of "gods") one. That is why it is misleading to simply say that Coptic grammar parallels the English usage of the indefinite article without noting, as you carefully do, that nouns with qualitative force (i.e. "unspecified quantities of a substance") "require an indefinite article in Coptic where there is none in English". That is a crucial difference between English and Coptic, as it directly pertains to the question of whether an indefinite article should be used in English in rendering John 1:1c. In other words, whereas the Coptic interpretation of the original Greek excludes a straightforward definite non-qualitative reading, it allows the other two semantic readings, and thus does not point uniquely to a single option. If the above is correct, the use of the indefinite article in Coptic would not uniquely support translating theos as "a god" as opposed to the various qualitative options. At the same time, it does not establish the alternative; the text would be ambiguous, as it is in Greek. However, it should suffice to point out that the Coptic would allow a qualitative reading that Harner and others have proposed as "most probable" for the original syntax of John 1:1c -- one that sees theos in that context as an abstract noun analogous to "truth", "life", "light", "love", i.e. theos is what defines the nature of the Logos, just as "love" and "light" define the nature of God, or "truth" defines the nature of God's word (cf. John 1:5, 17:17, 1 John 4:8).
Finally, one brief comment here:
I can't reproduce the Coptic fonts here, but you can see that he diagrams Coptic John 1:1c to say literally, "And was a god is the Word." Or in more smooth English, "And the Word was a god."
That's a fair way of expressing what Layton does, as long as it is clear that the "more smooth English" version is simply rearranging the syntax of the glosses and does not itself constitute a "translation" (no more than the lexical glosses themselves necessarily provide a translation when grouped together), nor does Layton provide the latter as his translation as well. It might be worth finding out why Layton makes the translation choices he does here.