So what might have been Elan's super secret plan?

[Peelee]

How is that a prequel? It was made after Fistfull of Dollars and For a Few Dollars more, and was focused on a Civil War campaign that actually happened. (No it was not a documentary).
Not seeing it as a prequel.

Well, as prequels tend to be made after the original film, and take place chronologically before the original film, and given that you readily admit that these two facts apply to TGTBATU, I'm more than a bit confused as to how you don't already acknowledge that it's a prequel.