D.M.
I wouldn't be too concerned about them not being comfortable sleeping on their back. My son, who is going to be 2 in July, was never comfortable sleeping on his back. When he was little, the only way we could get him to go to sleep (without being held) was to swaddle him and prop him up on his side. We got the idea from Children's Mercy. (He was in Children's Mercy for almost a week right after he was born and that is what they did.) He has never slept on his back. Even now, we lay him down on his back, but he always ends up on his stomach. That's just what's most comfortable for him. Once they start rolling over, you can't control the position they sleep in anymore. If you can't get them to sleep on their back, I would just recommend trying other positions.
I hope this helps!
D.