My background, in short, is that I graduated with a degree in Religious Studies and have had numerous discussions with members from a variety of religious traditions, including Jehovah's Witnesses. Only a lack of funding prevents me from furthering my education. In my studies I have come to see religion as a product of human culture rather than something which comes from on high. Thus I am currently an agnostic who remains unconvinced of the claims of "Truth" that the many religions have made. However, even as an agnostic, I think that there are many "truths" that the various expressions of religion throughout the world embody which are relevant to human life. In my studies I am particularly interested in Second-Temple Judaism and earliest Christianity as a product of the Second-Temple Period.
My answer to your question may not be satisfying, but I will give it anyways since you asked. You asked about Yahweh worshippers before Abraham and mentioned Melchizedek. Starting with Melchizedek, the text does not say he was a "priest of Yahweh" as you assert. Actually, the text calls him a "priest of El Elyon," i.e. a priest of the God El (cf. Gen 14:18). The text also says that Melchizedek blesses Abraham by El Elyon as well as El Elyon himself (Gen 14:19, 20). What is interesting here is that the Hebrew text gives the title of El as El Elyon qoneh shamayim wa-'arets (El Elyon, creator of heaven and earth). This is nearly the same as the title El qone 'arts (El creator of the earth) seen at Ugarit and a Hebrew ostracon from the 8th-7th century BCE which has [El] qn 'rts ([El] creator of the earth) (cf. Patrick D. Miller, "El, The Creator of Earth," Bulletin of the American Schools of Oriental Research 239 (1980): 43-46; other examples of this title are mentioned as well) Thus the text only shows Melchizedek to be a priest of El. Now it is true that in Abraham's reply the Masoretic Text has him say "Yahweh, El Elyon, creator of the heavens and the earth (yhwh El Elyon qoneh shamayim wa-'arets; Gen 14:22). It is clear, however, from the Bible's presentation of Melchizedek's use of the title, which lacks the mention of Yahweh, as well as the use of this title outside of the Bible for El in several sources that the name Yahweh has been interpolated into this title where it did not originally appear. Additional evidence for this may be seen in the Septuagint which reads ton theon ton hupsiton (God the Highest) which presupposes an original reading of simply El Elyon (BHS shows that the omission is also found in the Samaritan Pentateuch as well as presupposed in the "Genesis Apocryphon" (1QGenAp). However, von Gall's edition of the Samaritan Pentateuch, admittedly out of date, contradicts BHS in reading Elohim El Elyon).
As for the worship of Yahweh before Abraham, I said previously, support for the Exodus and the invasion of Canaan is not to be found in archaeological data. The same may be said for the Patriarchal narratives. Archaeological and Biblical congruity only begins around the time of Solomon (10th century BCE) and even this is much disputed territory, though I personally concede the historicity of Solomon and likely David as well, but not in all the glory the Bible ascribes to them. Thus asking me about "Yahweh worship before Abraham" is to assume the truth of the Biblical record on Abraham's existence which I cannot affirm. My position is spelled out rather clearly in my post where I note that evidence for Yahweh worship in Israel only begins in the mid-9th century and gradually comes to dominate Israelite religion from that time. The time of any historical Abraham would be centuries before this. Further I had stated that the origins of Yahweh worship itself is an issue which I need more study so that I cannot say how it was introduced into Israel nor who worshipped Yahweh before them, though a people in the northwestern Arabian peninsula are good candidates. So to answer your question: 1) I do not see that there was worship of Yahweh by the people of Israel before the 9th century BCE 2) Abraham is likely a literary/oral invention so that I cannot speak of worshippers of Yahweh before him.
Finally, you asked, "What point do you think that a god became a God?" Presumably what you are asking is when did Yahweh, one god who exists with other gods also existing beside him, become understood as the only deity which truly exists. Well this is asking at what date did monotheistic Yahwism supplant polytheistic or monolatrous forms of Yahwism. This is a difficult question, which is why I referred you to those books, but the rise of monotheism proper is best expressed in Deutero-Isaiah (40-55) which scholars date to the second half of the 6th century BCE and which emphatically denies the existance of any deities besides Yahweh. This monotheistic view of Yahweh appears to have been brought back with the exiles from Babylon to Israel. However, one should remember that not all Jews were taken into exile and so even at this time there were Jews who held to polytheistic/monolatrous forms of Yahwism. Thus in the 5th century BCE the Jews at Elephantine worshipped Yahu (Yahweh) along with Anat (who, as Baal's consort, essentially replaces Asherah's position in other Canaanite traditions with their prominence of Baal). This takes us into the Persian period which is the abyss of Second-Temple studies as there are few sources for this period and much room for discussion. When things pick up again in the Hellenistic period, late 4th century BCE onward, monotheism was pretty much the standard form of Yahwism.
Even in this context, however, one may find remnants of the older religion. Thus the "sons of El" who each ruled over a nation, which I mentioned earlier, are reimagined as a kind of "angel," yet their ruling over nations is still an accepted part of the religion, though in various ways. Thus Ben Sira echos the passage in Deuteronomy saying, "He appointed a ruler for every nation, but Israel is the Lord's own portion" (Sir 17:17). Jubilees speaks of how God placed the other nations under the authority of spirits to deceive them, but God himself rules over Israel (Jub 15:31, 32). Such a view is also what clearly stands behind the mention of the princes Persia and Greece in Daniel who fight against the divine being who appears to Daniel and is assisted by Michael who is said to stand over for the nation of Israel (Dan 10:13, 20, 21; 12:1). The War Scroll from Qumran speaks of God sending Michael "to exalt the dominion of Michael above all the gods (Heb. 'elim), and the dominion of Israel over all flesh" (1QM 17.5-8). This theme is even found in Rabbinic literature as well, where, for example, Pirke de-Rabbi Eliezer (8th century C.E.) speaks of the seventy angels who cast lots to decide rulership over the seventy nations (Pirke de-Rabbi Eliezer 24). This evidence, pulled from such diverse Jewish groups, attests to the widespread distribution of this idea as well as its antiquity which must have predated the rise of these diverse groups during the Second-Temple period and after.
So returning to the question, it would seem that monotheism as the view of Yahweh became normative sometime between the 5th-4th century BCE.