Two additional remarks on BeDuhn's quote by Earnest:
I find it amazing that BeDuhn seems to consider only the propriety of tense coordination in the target language, as if the "natural" character of the Greek expression was to be taken for granted. In principle, translation should aim at reflecting the style and level of language of the original, not to sound "smoother" or "more natural" than it is. And this can only be ascertained by comparing the particular wording of every text with general usage in the source language. Maybe BeDuhn does that elsewhere. But from a rapid survey, the overwhelming usage of Greek tense coordination is prin/pro + past tense in the circumstancial subordinate clause => past tense (perfect, aorist or imperfect) in the main clause. I found an interesting exception (which BeDuhn could have used) in Proverbs 8:25 LXX, "before (pro) the mountains were established (edrasthènai, passive aorist infinitive), before all the hills he begets (genna, present) me". But this appears to be a (meaningful?) exception: in v. 23 with a similar construction the main verb is in (past) aorist tense (ethemeliosen me), as in most cases.
Another interesting point is that BeDuhn does suggest that the use of the present tense implies continuous action (or state), even after Abraham came to existence, and finds it necessary to add "(since)" in his translation to render it, making his "I have been" a real present perfect. Which the NWT does not. I doubt the English NWT reader understands that. The French NWT reader definitely does not. In the original edition (1974), if memory serves, the verb was in the imperfect tense, Avant qu'Abraham vînt à l'existence, j'étais (I was). In the latest revision, apparently to mimick the English, we now have the passé composé, avant qu'Abraham vienne à l'existence, j'ai été. In both cases the main verb only implies being before Abraham, not since.