The Athlon used the same bus as the Alpha but that doesn’t mean it was an Alpha internally. I don’t know the specifics of what the RISC portion of the Athlon looks like but given the short time duration between the K6 and the Athlon, the Athlon almost certainly had to build on the K6.

so is Athlon based on a internal DEC Alpha RISC back end and a Next Gen translate code designed x86 front

The Athlon wasn’t entirely new, there was some K6 in it, but AMD did hire a lot of Alpha engineers (including Meyer) so they would have influenced the Athlon design.

is Athlon based on Next Gen and a further development of K6 or something entirely new? I know they hired Dick Meyer from Dec Alpha

As far as I can tell Greg Favor left AMD around the year 2000.