You haven't mentioned the number of stones and range of their size. Similarly, you have also not mentioned the details of polyps. Anyway.
1. The 'key investigation' for the diagnosis of both is ultrasound
2. The disappearance of stones without having caused any symptoms (particularly severe pain
) and spontaneous disappearance of polyps is unusual.
3. I would recommend you to repeat ultrasound scan from another experienced sonologist. I do not mean to doubt competency of your previous sonologist, but that's the best suggestion I can offer because of the fact discussed above (point number 2).