Jeez, I gave you the opposite answer to the one you wanted!
Sorry. Most people want it the way I described, so I gave you the autopilot/canned answer.
So as you were originally thinking, you do need to have PP answer <significantly> before GV grabs the call. Since the GV ring period isn't adjustable, then you will need to further shorten the PP ring period, if they have a way to do that. 20 seconds should work.
Since voicemail is one of the fundamental features of GV, there is no way to just disable or bypass it entirely.
Also, be sure you have DISabled GV's Call Screening feature, here:
https://www.google.com/voice#callsettingsIf call screening is ENabled, GV will never let another VM system take the message, as it requires a human to press "1" to accept the call.
Finally, GV also has some undocumented logic to try to detect if another VM system is answering the call, as will be the case when you are on a call on your PP line, and you don't answer the second call via call waiting. This feature can't be turned off. So, if you cannot get PP VM to take the call, no matter what you try, then you will have to decide: how important is it to you to use PP VM instead of GV VM? If you value PP VM more than any of the other features of GV, then you'll have to port your number out of GV and into PP.