I would not use a sleep positioner, I'm surprised a ped would say that, but then say to keep the crib free of obstructions? Anyway DD was (and still is) a big roller and mover in her sleep and often ended up on her tummy at that age. As pp said, I would make it safe as you can and then leave her be. And the sleepsack probably won't be a problem either, DD used one at that age and rolled all over, and now she can walk around her crib in the morning still wearing it, so if they want to move it doesn't appear to hinder them!
DD January 2008
DS September 2011