I only study biblical history as a hobby, but since no one else has attempted a response, I'll share my understanding with you.

I don't think there is any doubt in the current scholarship that Yahweh and El were originally different deities, but in my reading I've never come across anything that suggests that El is only ever loving and Yahweh is only ever fierce.

One thing that comes to mind is that the earlier narratives tend to involve a more personable god, one that walks and talks with the characters as when he scolds Cain or when he comes down to view Babel. The older god of the bible gets angry, he can't find things, has regrets, he changes his mind. Compare the earlier creation story of Genesis 2 where God walks around the Garden of Eden and makes clothes to the much later Genesis 1 where God merely speaks things into existence ("Then God said, “Let there be light”; and there was light."). There was a more fluid boundary in the older texts where God could come and go and actually resided in the tabernacle, contrasted to the later texts where God definitely lived "up there".

Another facet that I think applies here is as monotheism took hold God's nature necessarily changed. In the earlier texts nothing happened without it being God's will: If you followed God's commandments you were rewarded; if you didn't you were cursed. The God of the pre-exilic period was responsible for the good and the bad things that happened and if you suffered misfortune you deserved it. After the Babylonian exile, though, this thinking started to transform; when God's chosen people are repeatedly conquered there must be something more at play. The Book of Job is the most striking example of when this thinking shifted more towards God being a mysterious entity that you can't understand. Job's family and household are destroyed on a dare and his friends try to convince him that he must have done something to deserve such a punishment, and Job maintains his innocence.

Something else that molded the character of God in the bible is the influence of outside religions. If you can read it without Christian interpretation, there is no devil in the Old Testament. It wasn't until Judea was conquered by the Persians and later the Greeks that the Israelite religion adopted the dualistic notion that God is responsible for the good and the devil is responsible for evil. It is in this environment where the apocalyptic worldview developed: God has temporarily stepped away, the forces of evil are in charge, but God will soon return and set things right again. Enter Christianity.

I suppose my tl;dr would be that I suspect God's shift in personalities has more to do with external events (exile, Persian/Hellenistic influence) than the unique characteristics of the original deities.

Something that I think might interest you would be the Documentary Hypothesis, which is the theory that the first five books of the bible were originally 4+ documents woven together by a later editor. (There are scuffles assigning different verses and the dating, but that there were originally multiple sources is not in question.) The "J" source is from the southern kingdom and uses 'Yahweh', while the "E" source is from the northern kingdom and uses 'Elohim'. This online bible has been color-coded for the different sources.