Oh boy! A question about Greek AND Bible translations!
My B.A. is in classical languages and culture, and I am currently a Master’s student in religion.
NT Greek and Modern Greek exist on a continuum. Both are Greek, but over two millenia, the language has evolved and simplified. That is to say, Modern Greek is syntactically much simpler than Koine (the Greek of the New Testament). Some of the vocabulary has also shifted, with the meanings of some words having changed, and some loan words from other languages replacing more ancient words in common usage. A better comparison with English may well be the English of Chaucer compared to our language today. We can understand Middle English basically, but most people need a translation to read The Canterbury Tales well.
A competent translation (that is, by competent scholars who know the Biblical and related languages, and are historically informed) in any language is going to be accurate. The important factors are the competence of the translators, their methodology, and the manuscript choice, not the target language. In general, the translations I trust the most and use most regularly are the RSV/ESV (devotionally), the NRSV, the NJB, and the REB (these latter three, academically).
I think the approach of challenging the New World Translation, or even directly challenging the beliefs with an “I’m right, you’re wrong” approach might close off the possibility for changing a JW salesperson’s mind. I have found, in personal experience, that more progress is made by starting with common points of agreement and building from there, rather than starting at a point of disagreement. However, the NWT rendition of “the Word was a god” is grammatically illogical. The only theos (God) mentioned in the previous phrase has the definite article attached to it; kai (and) is a logical connector. No new noun has been introduced, so logically the theos in the line “kai theos en ho logos” must refer back to the prior phrase’s “ton theon”, which includes the definite article.
Take into consideration as well that Jn 1:1 refers to Gn 1, the relevant parts of which read in the Septuagint, “En arche epoiesen ho theos ton ouranon kai ten gen. […] eipen ho theos Genetheto phos” (Gn 1:1, 3). eipen means “said”. It is from this that the Evangelist gets ho logos, that is, the word, which is spoken. The logic of the passage in question not only links the Word to God, but identifies said Word as the very same God who was “in the beginning” in Gn 1:1.
I hope this has been of some, even if minor, help.