5'1, 100 lbs. Over the muscle 250cc high profile implant to under the muscle; what are my options?

If you know how to loosen the muscle properly going from over to under should not give you a double bubble if you don't have one now. I think a 150 hp implant would look kind of round. If you want to look more natural a lower profile implant is often better in that size range. If they are firm you may have a bit of capsular contracture that is making them look round

I am sorry to hear about your concerns after breast augmentation surgery. When it comes to revisionary breast surgery, it is extremely important to make the correct diagnosis, in order to formulate a surgical plan to improve the patient's specific "problem".In your case, given your description of "very firm", I would be concerned that capsular contraction is present. If this is the case, conversion to the sub muscular pocket plus/minus any additional maneuvers (such as breast lifting) will likely be in your best interests. You may benefit from consultation with additional board-certified plastic surgeons who can demonstrate significant experience helping patients with revisionary breast surgery. Best wishes.