I totally feel you! I am dying over here wondering what my baby is. Some friends were trying to talk me out of finding out and waiting until the birth. That is way too much torture for me. Waiting until 20 weeks is long enough.
As for sleeping on your back. It causes more pressure on your vena cava artery in your body causing restricted blood flow for you and baby. I don't think it puts any pressure on the cord though. I would sleep on your back as long as it is still comfortable. When it's causing you problems, you'll know it and work out another sleeping position. Hopefully it won't cause much pressure until the baby is much bigger :)
I can't imagine how nerve wracking it must be not to have a u/s yet; especially after a previous loss!! If it will give you peace of mind and you can find an affordable place, I say go for it and try to get a peak. I was in my dr's office last week (because of severe cramping) and at 15w got a u/s. I asked if there was any way to tell if boy or girl and tech said no, they all still look like boys at that point.
As for sleeping on your back . . . I have the same concern. I have read where it is best to sleep on your left side as that allows for the greatest amount of blood flow to the fetus. I've had lots of ladies respond to my posts with varying advice. Some say that their dr's say as long as you are comfortable it doesn't matter. I also read, that even if you are sitting up just a little bit, you should fine - like if you are watching a movie and are gently propped up.
I've been trying very hard to sleep on my side, but have a lot of hip pain and often find myself rolling over onto my back. I'm guessing sooner or later it will be too uncomfortable for us to sleep on our backs and our bodies will do what it's naturally supposed to do during pg. I also have a body pillow, and I've heard that propping pillows around your body can help alot.