There's a principal in science that a strong hypothesis doesn't depend on numerous improbable factors. I read that somewhere, I think it was Phil Plait who created the bad astronomy website, but I can't find it right now.
For Mercury to have done what you claim, a 2nd planet would have needed to form between Earth and Jupiter, OK, maybe that happened, in fact, the young solar system was probably full of protoplanets.
This planet would then have needed to change it's orbit. This usually takes a more massive planet to toss the smaller one, but there's two problems here. Could Mars push Mercury all the way into Earth's orbit? Probably not. Could Jupiter? Sure, but then, where did Mercury get all it's Iron if it formed between Mars and Jupiter. It's becoming problematic.
Planet on planet collisions are rare, obviously not impossible.
Then there's the problem of how did the collison result in Mercury flying off, losing some material, the moon forming and Earth rapidly rotating. Planet on planet collisions tend not to be glancing blows because planets are too big, and gravitational wells are pretty deep. I don't want to say it's impossible but that's an odd scenario which seems highly unlikely.
And then, Mercury changes it's orbit again, perhaps gravitational assist from Venus and it ends up where it is.
A hypothesis like that depends on that many rare events all happening isn't not going to be a popular one. Building a hypothesis on a single improbable event is fine. The giant impact theory began like that.
A good hypothesis also has evidence behind it. Mercury looks like it formed about where it is now, clearing out it's orbital neighborhood and being made up of denser material that was more likely to orbit closer to the sun. There's very little reason to stand behind the Mercury impacted Earth idea and there's no evidence to support it.