Why I think Robotnik won't be an alternate universe version of Eggman.
I really don't have ab exact how for his survival in Sonic 3 but Doctor Eggman has survived many things that would've easily killed someone on the Sonic the Hedgehog franchise, so and while he shown some sense of humanity before his sacrifice that won't mean he'll be reformed like that, he's helped save the world in Sonic Adventure 2 and went right back to being a villain in the very next game, and in other games and the 3rd movie he has teamed up with Sonic sometimes only to betray him afterward and finish the game as the villain. So I really don't think his sacrifice in the 3rd movie leads to an alternate version of Robotnik as the only way to make him the villain of the 4th film.