I am seeing a lot of Americans every time I am in Cuba. You have to remember that it's USA who has issues with Cuba. Cuba doesn't have any issues with USA. You will be greated with a smile and respect. Expect a lot of anti-american jokes against your government though.
Most travel through Mexico or Toronto as these routes seems to be the cheapest options.
All you have to do is ask the customs agent not to stamp your passport. They actually know this and won't do it,but you should still ask. It's very safe and thousands go through this every year. I've never heard of anyone ever having any problems upon return.