No offense to this theory or to any of you who believe it's true, but get this through from the very first episode, He's a Human from another Dimension! I don't think any of his ancestors ever set foot on to any other Dimension.
Like really he just a regular kid from earth, he even said it in "Camping trip" when he wanted to show Star the Geyser Old Youthful because She showed him things that he didn't know they even existed. Which I don't think even knew that other Dimension even exist either
How you think it would've been if it was Ferguson or Alfonzo or Oskar or any other person who was chosen to guide Star that's NOT Marco.
Even in the Comic of "Deep trouble issue #2: Ole Moon River" (which is also a flashback to a previous love triangle of Moon and River) It even proves that they go back in time and can alter "Mewni's" timeline, not earth's because Marco will still exist and the only timeline his will change is he wouldn't meet Star and he would've still been a safe kid with no one to help his confidence in life.
Plus the whole cheek mark thing was just a side effect from the use of the Magic wand. I've bet even Ludo has some underneath all his feathers, Or he just don't because it was Toffee who was just controlling and corrupting the magic.