Thank you for your question and for sharing your history and photos. Based on your picture, I strongly recommend considering a bra line lift, also known as a back lift or horizontal torsoplasty.
Despite the potential benefits of aggressive liposuction, you might still be left with loose skin...
I encourage you to contact your surgeon's office to discuss your concerns. There seems to be an area where the tissue may be compromised a bit. As far as sleeping on your back, each surgeon has her/his own post operative protocol. Your surgeon is most familiar with the details of your surgery...
From your submitted photo the best option is an upper back lift. This is the best way to address the skin laxity and give you the best contour. It does leave the incision across the back, but it is placed to help it be hidden by undergarments.