No. Rolling over is a normal milestone in development. It is very important to make your baby´s sleep area safe. Never let a baby sellep in a bed as he might roll off and seriously injure himself. Take everything out of the crib and avoid positioners and crib bumpers as advised by the american academy of pediatrics. Avoid bed sharing, it is not "quality" time with your baby and has risks.
No. Once your child is old enough to roll over in her sleep, we don't typically worry about sleep position.
No. A young baby (who is young enough to get sids) isn't able to roll over yet. A young baby is put to sleep facing up, and should still be facing up when he wakes up. If a younger baby, for whatever reason, is found on his tummy in the crib, just flip him onto his back. An older baby is strong enough to roll over and end up on his tummy, but since he is older, he is at little or no risk of sids.
No. Once babies roll, their sids risk goes down generally, since they are able to move with some purpose. However, there cannot be anything else in the crib, including blankets, bumpers, pillows, stuffed animals, or sleep positioners. And of course, no co-sleeping. There isn't any safe way to get them to stop, including sleep positioners specifically, so let them do it and hope for the best.