Rob,
Let me answer your questions and then I will give you an update on my progress.
1. S3 vs. S4 See http://www.marketnews.ca/Hands-onReviews/Hands-onReview:BlueAntSupertooth3vs.BlueAntS4.html , but, in short, the S4 is true hands free device that accepts a bunch of voice commands.
2. and 3. I have been unable to pair the phone with the Pioneer and the S4 at the same time, so these questions are irrelevant.
So, before I yanked the head unit out of my truck and sent it off to Long Beach, I called my friend who has two PhDs (one in Physics and one in Electrical Engineering) and