I believe it is better that you do not mix the NT and OT when analysing something like this. One reason is they are two very different languages.
Also I would like to ask you to analyse from the language, and some experts in the language, not apologists for the Christian doctrine.
I have heard from Christians that "If Elohim were exclusively singular, this would read yachid which can only mean one." Yes, Echad does in fact have a compound singularity in it, such as having one synagogue with a hundred people inside." Thats nice apologetics but absolutely bad logic. I can say "one country" and that would mean a 100 million people in it but its still one country. "ONE MAN" is just one man. So one is not a compound one every time, but it would depend on one persons baggage that he is already carrying. If he really really wants to make God more than one, and that's all his faith is, then he would want to make God like a country. One means one country with many people.
But linguistically its absurd. Because when you think of God in this sentence its singular. One.
What apologists do is that they try to conflate the word one of many contexts into one. As in, when the Bible says "The people are one" it is the same saying "God is one". But that's bogus because in this particular sentence or in other places in the Tanakh God is always singular and is never referred to as "people". How could YHWH (Sorry if the mention offends any Jews who are reading this) which is used as a proper name be like a group of people and/or speaks in "ONE" language and thus this one is a compound one? It is a very bad but valiant attempt. Think of the verse Genesis 11:6 where the same word Echad is used. It says "the people are one and the ALL speak one language". DO you see that this verse says "People" and it also says "ALL speak ONE language". Using this type of verses to say this one is a compound one has to be a dishonest attempt because one cannot be so illogical possibly.
I dont think one has to be a language expert to understand this. I am no expert in Hebrew at all. Yet, what I do know is that some people like Daniel Pipes have tried to use the same logic with the arabic word Ahad saying one is "Wahid" but not Ahad. The same logic. When someone is that ignorant, it is most likely that this person is dishonest. I would understand a layman on the internet believing someone and repeating something like that, but educated people in the subject cannot be so daft. Thats impossible.
Equally unreasonable is the suggestion of Michael Brown on Zechariah 11:8, where the prophet speaks of one (echad) month. Brown asks, "What does that tell us about the essential nature of a month? Does it mean that a month does not have thirty days because it is one?" The word "one" modifying "month" is not remotely connected to how many days there are in a month! On Brown's argument the word "one" loses its fixed sense as "one single." And the whole argument is then brought to bear on the central question of monotheism and is used to justify a plurality in the Godhead.
How would the proponents of one as "compound one" explain Nehemiah 11:1: "one (echad) out of ten"? Or Ezra 10:13: "one* (echad) day or two"? "Two are better than one (echad)" (Ecc. 4:9). "If two lie down together they keep warm, but how can one alone[echad] keep warm?" (Ecc. 4:11). "Where a lone [echad] man may be overcome, two together may resist" (Ecc. 4: 12). The rest of the 970 appearances of echad might be cited to make exactly the same point.
Haha. With this kind of logic, the word one is not meaning one because we make sentences like "Year one". But the year has 365 days.
Anyway, its an interesting topic. So that's what I have to say on it.
Cheers.