I made it quite clear in another thread that it was translated this way numerous times by non-JWs, centuries before the NWT and that it's a common fallacy, if not outright deliberate deception that it's taught by churches and scholars that the NWT was the first and only to do this.
An important point, however, is not simply that the NWT translates John 1:1ff in this way, but that they do so inconsistently:
Yet they did not change other verses in a similar way but only when it suited their doctrine, if it was with some sort of consistancy it would probably be more acceptable.
You mentioned Wallace, who does argue that theos is "qualitative" but also states that "The grammatical argumeht that the PN [predicate nominative] is indefinite is weak. Often, those who argue for such a view (in particular, the translators of the NWT) do so on the basis that the term is anarthrous. Yet they are inconsistent..." p. 267 of Wallace's Greek Grammar
He goes on to quote R.H. Countess: "In the New Testament there are 282 occurrences of the anarthrous theos. At sixteen places NWT has either a god, god, gods, or godly. Sixteen out of 282 means that the translators were faithful to their translation only six percent of the time..."
and again: "The first section of John-1:1-18-furnishes a lucid example of NWT arbitrary dogmatism. Theos occurs eight times-verses 1, 2, 6, 12, 13, 19- and has the article only twice-verses 1,2. Yet NWT sex times translated "God," once "a god," and once "the god."