Well that's the problem, where in the movie(s) does it say that everyone knew what exactly happened to Anakin?

You may think that it SHOULD be this way, because I mean he was a big pilot and all, but the fact that Luke had worked with the rebels for a long time and yet no one told him, is actually a pretty good indication that no one knew Anakin had turned to the dark side.
Leia had been leading the rebels with Vader in the background for how long before IV? How big was her surprise that Vader was Luke's father?

So the "problem" you're trying to "solve" here, is non-existent.
The turn probably happened in secret, and no one in the OT knew, aside from Ben, the Emperor, and maybe some FEW Imperials (but doubtfully).

Having that said, in case you think it's way too suspicious that Anakin just disappeared and Vader appeared, there is still a better alternative:

Vader still killed Anakin, but he's Luke's father. Luke's been taken away from Vader and adopted by Anakin.
A bit like in the Wolverine intro.