What I meant to point out is that the WT did not really settle on any one "correct" exegesis of John 20:28; this itself is characteristic if you think of it. They are absolutely assertive about the one and only "correct" understanding of their own prooftexts (they would never admit, for instance, that the "other sheep" in John 10:16 could also refer to the Gentiles). But when it comes to "anti-prooftexts" so to say, i.e. texts they cannot argue from but have to explain away, they do not "lock" the interpretation.
This imo is useful tactics in several ways: (1) in the defensive part of a doctrinal discussion they appear less dogmatic, hence more "reasonable" than their adversaries; (2) the audience gets the (wrong) impression that several interpretive hypotheses among which they do not choose are somehow stronger than the one they oppose (a fallacy, since logically only one of them can be "right"); (3) it characteries the "anti-prooftexts" as difficult or obscure, and move them to the periphery of the debate (contrary to their prooftexts which they treat as if they were crystal-clear), even if they are actually climactic in the text (as is the case of John 20:28, especially when you consider that the book once ended there, whence the inclusio with 1:1).
The three interpretations of John 20:28 which the WT retains as possible without choosing between them are, in effect: (1) "Jesus, my lord! Jehovah, my God!" (an exclamation addressed to two distinct persons, which sounds overly farfetched even for a NWT reader); (2) "Jesus, my lord and my god" (in principle ruled out by the capitalisation of "God" in the NWT, but still mentioned as a possible explanation by WT literature); (3) "Jesus, my Lord and my God" implying that the Father (= Jehovah in WT doctrine) is addressed through Jesus (not besides Jesus as in option # 1).
Option # 3 is not that far from the Johannine perspective, especially as developed in 14:6ff:
Thomas (!) said to him, "Lord, we do not know where you are going. How can we know the way?" Jesus said to him, "I am the way, and the truth, and the life. No one comes to the Father except through me. If you know me, you will know my Father also. From now on you do know him and have seen him."
Philip said to him, "Lord, show us the Father, and we will be satisfied." Jesus said to him, "Have I been with you all this time, Philip, and you still do not know me? Whoever has seen me has seen the Father. How can you say, 'Show us the Father'? Do you not believe that I am in the Father and the Father is in me?
N.B.: Jesus is not the Father but the Father is in Jesus, hence reached, known and seen through Jesus.
It is not at all impossible imo that the scene in chapter 20 portrays Thomas as finally "seeing" the Father (= God) in Jesus (whom he already addressed as Lord in chapter 14). This is an important nuance, which differs from both (neo-)Arian (including JW) and Trinitarian uses of this text.
[Unrelated detail, I'm nitpicking only because you come across as very assertive: it is fairly meaningless to say that "the Catholics regard verse 28 (not John 1:1) as "a literary inclusion with the first verse of the gospel: “and the Word was God” (NAB notes John 20, 28)". Catholic official dogma does not "lock" exegesis as your wording implies. Catholic scholars are largely free to discuss and disagree about the original meanings of particular texts, a fortiori broader literary consideration such as the inclusio structure (with which I agree in that particular case), even in books which are granted the official imprimatur, and the church is not officially committed to their views.]