dear Viviane...
you said, "El (the supreme God) and Yahweh are distinct and different gods, along with Asherah, who was ALSO worshiped. El, in proto-Hebrew Semitic religion (from which the Hebrew religion sprang), El Elyon was dad and Yahweh and Ba'al were brothers... so, exactly what I said. Yahweh is their national God, the one they worship FIRST, but they also worship many others."...
The religion of the hebrews sprang from encounters with God. In their efforts to conceptualize for themselves Who this God they encountered was, what He did, what He would do etc. they drew on local mythologies to express themselves. The God they encountered did exist prior to the local god Yahweh (as JD pointed out with reference to Melchizadek, Abraham and jacob). The God they encountered is creator of earth, God of the earth and He chose the hebrews. Some of these actions in time resemble actions attributed to a local god named Yahweh...that NAME and what it means is a perfect expression of who their one God is. That's how Yahweh can pre-date the semetic religion from which he sprang.
one reason I think that the commandment to worship no other God before Me means in my presence or in front of me (instead of your interpretation, worship Me as the highest God of many gods you can worship) is in the israelite sacrificial system itself. The sacrifices and offerings that were required throughout the year appear to be something that is required by other gods as a means of worship...a god of the harvest, a god of livestock...a god of tribal protection/preservation etc. All those "things" that were distributed to many gods by other tribes for gain were to be given to one God.
love michelle