(warning will contain spoilers if you click the + button!!!): I just finished reading decently far ahead and honestly I've still got no clue about MC's past. They kinda clarified it, the two different "El"s and all, but also not really. You might be on to something with the first half of your conjuncture because taking in past events it does seem like MCs birth as Elqueeness was halted. They brushed it off as the Demon king's work with his ploy of drinking children's blood and all. There's still more that could be uncovered if the author wanted to go there. But the story keeps going off on tangents which are related and slowly, extreamly slowly unravel parts about the underlying plot going on. So it's possible that there's more to uncover with MC's identity and the previous El that may or may not have existed. They spent so much time.on it in the next 20ish chapters, yet the conclusion was just as vague as the beginning ¯_(ツ)_/¯. So I got no clue.
As for the second part, ya, I don't think so. They've already said that Rucariem is the demon god's messenger, I'm pretty sure that's accurate, although it does look like he has his own agenda kind of going on. The demon god is screwing around somewhere else right now. You'll meet him in a few months probably (with current translation rates)
Seems like we can just wait and see... (〜￣△￣)〜
Yeah didnt the demon king say Ruc contracted the demon god?
And the plot THICKENSSS!!! Is El the real body of the god of all gods? that would explain how the existence of the first El was basically erased and how he was reborn wrongly as the korean boy and became Elqueeness afterwards! And I have a feeling that the demon Rucariem is actually the demon god or at least a part of him.