I played around with your PV-extension idea. I have a similar extension in Alaric but wanted to try your way. I interpret it as "every 6 ply with only PV-moves do extend". That means after 12 ply with only PV-moves you will extend again.
Now I read something you said that puzzled me. "it happens only a few times in the late ending that eventually 2 plies are extended" Your post
But I suppose that ProDeo goes deeper than 12 ply in many positions in 2-3 minutes?
Maybe I misunderstand something here.

/Peter