Hi Tenacious!
I will answer the 2nd question first:
Question 2:
Now, I'll address (not really me but I'm definitely in agreement) the validity of Colwell's Rule as selectively applied by NWT translators in order to fit their theology and take away the Deity of Christ. The pressing question is posed by ChristianCourier.com:
“Why is it that the word ‘God’ is translated as ‘a god’ in John 1:1b and 18b of the New World Translation, and yet the identical construction is rendered as ‘God’ in verses 2, 6, 12, and 13 in the same context?”
The criticism above has been prominently championed by Robert H. Countess (Presbyterian), and repeated by legions on the internet. But Countess failed to mention the following:
The "a god expression" in verse 1c appears in a simple nominative clause. The other texts are not.
Verse 2 has God preceded by the definite article, "the God." So, it rightly appears as "God" in the English versions.
Verse 6 has God in the genitive form, the "of" case.
Verse 12 has God in the genitive form.
Verse 13 has God in the genitive form.
A Grammar book explains: “The use of prepositions, possessive and demonstrative pronouns, and the genitive case also tends to make a word definite. At such times, even if the article is not used, the object is already distinctly indicated.” (A Manual Grammar of the Greek New Testament, p. 137. Bold & italics added.)
The same idea is expressed by Daniel B. Wallace in the Grammar mentioned below, who also adds:
“The most likely candidate for θεός [at John 1:1c] is qualitative.” (Greek Grammar Beyond the Basics, by Daniel B. Wallace, Zondervan Corporation)
Question 1: The ESV does indeed translate the verse you cite as "No doubt this man is a murderer." But still yet, the ESV also translates John 1:1b as "and the Word was God." So why did the ESV choose not to translate it as "a god"?
Most Bible translations (ESV and NWT being no exceptions) are done by religious individuals who hold specific beliefs, like the Trinity, immortal soul, hellfire, etc., or the opposite. That is, Evangelicals, like the Witnesses have their own agendas as well, if altogether different.
The ESV makes it plain in their Preface that their version is done by trinitarian evangelical believers for orthodox church-goers:
“The doctrinal perspective of the ESV Study Bible is that of classical evangelical orthodoxy, in the historic stream of the Reformation. The notes are written...within the broad tradition of evangelical orthodoxy, the notes have sought to represent fairly the various evangelical positions on disputed topics…..” (Introduction, p. 10, ©2008) “All [the scholars and advisors involved] are committed to historic Christian orthodoxy. […] “And so to our triune God (Father, Son, and Holy Spirit), and to his people, we offer our work ….” (p. 12)
Now in answer to question one, Acts 28.4 does not address the deity of Christ, since Paul is the referent. Thus, the translators had no issue applying customary grammatical principles to the clause (NOT a rule, though.) However, at John 1.1 they face the difficulty of whether to describe Jesus simply, qualitatively as "divine," or go further and assuage the reader to believe that Jesus is more than a god or divine, that he must be "God" himself. Hence, their final reading. In the end, our doctrinal preference will dictate which version is preferable.