This idea involves there being a bad guy called “Darth Vader” already in place as a separate entity from anakin.
Three way volcano/mustafar duel between Obi-Wan, dark/turned anakin, and Vader. There is a situation like in ROTS where they are fueling on a platform that is falling/sinking into the lava. Obi wan gets tossed aside. And watches as anakin and Vader fall seemingly to their deaths. It would appear almost as though anakin sacrificed himself for Obi wan.
Later in the film, we see a charred hand pulling themselves out of the lava. The body is unrecognizeable.
We can then get the “birth of Vader” scene minus all the talk of padme.
The audience would assume the figure to be Vader when in reality it was anakin. He ceased to be anakin skywalker and “became” Darth Vader. In this incarnation “became” means he took up the mantle. Kind of like the dread pirate Roberts in the princess bride.