There is so much to be said about the case you are trying to present here. The basic problem is that you are concluding too much from the evidence. You have sufficient evidence to show that the OT was incomplete and that God was incompletely revealed in it, but you certainly don’t have sufficient evidence to show that it wasn’t the “true God” who was revealed there.
What you neglected to include was the fact that God invited those he did to come up to the mountain, but also distinctly allowed ONLY Moses to “come near.”
Moses alone shall come near the Lord; but the others shall not come near, and the people shall not come up with him. (Ex 24:2)
In fact, the mountain was covered in cloud and he only spoke to Moses from the cloud: "…he called to Moses out of the cloud." (Ex 24:16)
Clearly, Moses himself does NOT see God “face to face” at that point because in Ex 33:17-23 God explicitly states “…you cannot see my face; for no one shall see me and live.” and …” you shall see my back; but my face shall NOT be seen."
So in the earlier chapters when you say in “…Exodus 24:9-10 Then Moses went up, also Aaron…and they SAW THE GOD OF ISRAEL…” (your words,) what has to be meant is something like ‘knew with certainty that it was God who was present’. In other words, not even Moses saw God face to face because he longed to do so but was denied. He only saw God’s “back,” which means MOST CERTAINLY that the rest of the people didn’t even see that much. What they saw was the cloud that God was within on the mountain.
Again, explicitly denied in Ex 33:17-23.: "…you cannot see my face; for no one shall see me and live.”
Notice who did the “knowing” in the passage: the Lord.
Never since has there arisen a prophet in Israel like Moses, whom the Lord knew face to face. (Deut 34:10)
It doesn’t say Moses knew the Lord “face to face,” it says the Lord knew Moses “face to face.” Nor does it say they knew each other face to face.
So all of these are actually consistent with what the OT says about seeing the face of God. No one, not even Moses or Elijah did.