I believe a writer begins with their message in mind and then composes that message from the top down, not the bottom up. This means individual phrases and the use of grammar are tools used to convey and support the message. Individual statements and the grammar employed must be approached in this light; when one encounters a statement which can be understood in more than one way, it should not be understood to contradict the message.
Paul makes a statement which can be understood as expressing the deity of Jesus:
To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen. (ESV)
In addition to potential to understand the phrase as the ESV does, Paul's use of ὁ ὢν recalls the divine Name Moses is to give to the Israelites which the LXX renders as ὁ ὢν (cf. Exodus 3:14). Unless one believes Paul was careless, the rule of composition demands a statement of deity. Moreover, John's use of ὁ ὢν in the Prologue, written after Roman's must be seen in the same light. If John did not accept Paul's use, he would not have used it to introduce the Gospel.
Composition vs Grammar
The importance of composition and grammar is how the Fourth Gospel begins:
In the beginning was the Word and the Word was with God and the Word was God.
As if to thrown down the gauntlet, John opens with a statement involving the Word and theos. Does he intend to say "the Word was God" or "the Word was a god." Either is possible and the debate on the grammar centers on which is stronger. Yet, the best way to understand the writer's intention is to examine a grammatical question in the light of how the passage was composed.
If "a god" was intended there should not be anything to suggest the Word pre-existed God: in the beginning was the Word. Instead there should be no implication of pre-existence: in the beginning the Word was with God. In addition, if one did not intend to express deity, one would reverse the order placing God before the Word. Yet the actual verse reflects neither of these simple uses of composition which would resolve the grammatical question of God/a god.
Finally, if the actual composition implies pre-existence of the Word and follows with a second statement implying equality and possibly superiority of the Word to God, but did not intend the reader to make a wrong connection, then the writer should compose a statement which explicitly makes their point: but the Word was not God.
The fact the writer created the implied pre-existence and equality/superiority of the Word and finished the opening statement by failing to resolve the implication is an argument from composition for the Word was God. At that point the reader is left to consider, what type of God was the Word? Is He God of languages, or wisdom, or knowledge?
The writer soon composes another statement with both Word and God which the reader might understand as an OT reference to the Word of God:
6 There was a man sent from God, whose name was John. 7 He came as a witness, to bear witness about the light, that all might believe through him. 8 He was not the light, but came to bear witness about the light.
This is a typical historical statement about God sending a prophet to deliver a message. God and Word are present in the passage. God is explicit; word is implied. John received the word from God to deliver the word/message. There is no ambiguity or controversy in understanding the passage. However, in composing this statement the writer chose to write theos not ho theos. In other words, the writer omitted the article which would clearly identify theos as God and composed a statement which, once again, implies the Word is theos.
The grammatical conundrum of the first verse should be understood consistently with verse 6. If one believes the Word was a god based on the significance of the use of the article in verse 1, then one should continue with that understanding, there was a man sent from a god (i.e. the Word)... On the other hand, if one believes it was "God" who sent John, despite the purposeful omission of the article, then one should understand verse 1 consistently. Finally, from the point of composition, verse 6 is also ambiguous. One expects to learn it was ho theos who sent John, but instead encounters theos, which implies the Word. Yet after reading the entire Gospel, a reader is able to resolve both ambiguities from a single point of view. I and the Father are one...know and understand that the Father is in me and I am in the Father (John 10:30, 38).
ὁ ὢν in the Prologue
From the perspective of the historical Jesus, the Prologue is an unnecessary addition to the Gospel. From the perspective of the Christian Church, the Prologue is essential to understand the logical questions regarding God's plan of salvation. What was there a prior relationship between Jesus and God? If so, when did it begin and what was the nature? Is Paul's teaching which consistently and in creatively different ways imply the deity Jesus, in agreement with what Jesus Himself taught? In this regard, composing καὶ θεὸς ἦν ὁ λόγος virtually demands and the Word was God, for if John intended to refute those who misunderstood Paul, one expects an explicit answer: and the Word was not God.
Obviously, John never wrote to explicitly put an end to the question of the deity of Jesus Paul had created. In fact, those same implications are consistently found throughout the Fourth Gospel. From the perspective of composition, John picked up where Paul left off. Again, this argues for and the Word was God. Yet, the deniers may fall back to the more tenuous position that the Word is not Jesus. Here the composition of the Prologue refutes that position:
A: The Word with God (1-2)
B: The Word's role in creation (3)
C: God's grace to mankind (4-5)
D: Witness of John the Baptist (6-8)
E: The Incarnation of the Word (9-11)
X: Saving faith in the Incarnate Word (12-13)
E': The Incarnation of the Word (14)
D': Witness of John the Baptist (15)
C': God's grace to mankind (16)
B': The Word's role in re-creation (17)
A': The Word with God the Father (18)
The main point of the Prologue is becoming children of God. It was composed to be in the center of the structure, surrounded with an equal number of points before and after. Again, composition conveys the message. The pre-existence of the Word and role in creation are of secondary importance. They are necessary to affirm the Word's ability to continually create all things. The reader knows they have the authority to become a child of God because the Word has the ability to create children of God regardless of their original ancestry. By His Name only does one obtain eternal life (cf. Acts 4:12).
Verses 1-2 and 18 were composed as complementary points in support of making children of God:
1 In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God.
18 No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him.
First, God is connected with Father by composition and, once again, the Word is connected to God, μονογενὴς θεὸς. Second, the writer's use of ὁ ὢν recalls both Paul's use in Romans and the LXX in Exodus.
2Third, the logical progression of composition works in harmony with the message of becoming children of God. It is only after the message of becoming children of God is the term "Father" used. As if to say, those who are children of God, know God as Father (cf. Romans 8:15, Galatians 4:6). The Prologue has taken Paul's ὁ ὢν to a higher level.
It is incomprehensible to consider these words and this structure were composed unintentionally and now mistakenly lead the reader to believe John resolved the questions Paul raised by denying the deity of Jesus.
Rather, the consistent and intentional use of composition resolves every potential grammatical ambiguity in favor of "God."
1. This answer discusses the chiasmus of the Prologue.
2. There is a question of original text. One variant is ὁ μονογενὴς υἱός ὁ ὢν (TR and MT) the other is μονογενὴς θεὸς ὁ ὢν (CT). There is no question of the authenticity of ὁ ὢν. Additionally, if Son, υἱός was intended, the article ὁ μονογενὴς, the monogenēs, is redundant and appears to be an addition to distinguish Jesus from Luke's use of the term.