Two of the disadvantages of sub muscular augmentation that are rarely addressed, are that the implants will tend to be displaced a little more to the side when under the muscle, and in some cases the extra padding of the muscle may actually result in loss of definition of of the implant and thus the augmented breast. Contrary to many peoples beliefs, under the muscle does not always look better than over the muscle. Looking at your photographs, this appears to me to be the case. I believe your breasts are already naturally somewhat far apart, and in your case the implants under the muscle does not help. Over the muscle will allow the implants to be placed a little closer together, the muscle will no displace the implants, and you will likely have better breast definition towards the cleavage area. Unfortunately there are also potential disadvantages of sub glandular augmentation such as increased risk of capsular contracture, and possibly increased visible rippling particularly with saline implants. There is no right and wrong, just pros and cons that you should discuss with your surgeon.
Often I recommend fat grafting towards the middle to help augment the cleavage - know as composite breast augmentation - but from the pictures it is hard to assess whether that would be appropriate for you because of the poor breast definition.
Adipose Derived Stem Cells are some of the small cells that live around the tiny blood vessels in fat, and help regulate the day to day survival of fat in your body and changes as you gain or lose weight. They are essentially the healing cells found around small blood vessels everywhere in your body, but they are much easier to isolate in larger numbers from fat stores with liposuction. They help regulate the survival of fat when it is transferred and as you say support the transferred fat. There is evidence that adding extra stem cells to a fat graft by isolating them from a separate sample of fat can help the survival of a fat graft, but for the most part this is not practical in large volume fat grafting like the breast, because a lot of fat volume is wasted when separating additional stem cells to add. These cells are likely the cells that help restore normal tissue in scarred areas, and work in conjunction with the actual fat cells (adipose cells) to help replace scar tissue with normal tissue which contains fat. Te changes in the ADRC's, differentiation etc is a complex, still incompletely understood process, but it is well accepted that they are essential to the survival of a fat transfer. The cells already exist at the time of transfer, so there is not time lag, nevertheless they will differentiate to different functional cells at different times depending on what they are needed for - this is the essential property of these cells that allows them to change into whatever is needed, such as cells to build new blood vessels, or cells to make new fat cells.Various factors cause these cells to essentially differentiate and go to work when fat transfer is done, primarily things like low oxygen supply to the newly transferred fat etc.
There is a lot of misunderstanding about the concept of a double bubble. Most likely there is a slight discrepancy between your original breast shape and the shape of the implant with your own breast being a little narrower in the area of concern than the round base of the implant. This is a common problem as breast implants are made to fit an ideal breast shape, not all variations of breast shape, particularly breasts with what we call a narrow lower pole, or sometimes what we call a mild moderate or severe tubular breast shape. So what you are seeing is a step off between the edge of your original breast, and the new edge of the implant which extends further out. In most cases this step off improves over time as the implant stretches the tissues, but sometimes a perceptible step persists as in your case. I am almost always able to counsel my patients ahead of time that this may result by evaluating their original breast shape and explaining to them this mismatch that may occur. My approach to the problem is to discuss the possibility of fat transfer in the area, either at the time of the original procedure or as a second procedure. In this way we can build up the area where the breast is deficient to smooth out this step and create a rounder breast shape. This is known as composite breast augmentation.
I always remind breast implant patients, that breast pain is not necessarily related to their implants and frequently is due to the breast tissue itself. Mastodynia, or breast pain, occurs in women who do not have implants as well. The most common cause is simply the size and weight of the breasts. In your case, you describe your breasts as not feeling hard or firm so it is not likely that you have pain due to capsular contracture. It is important that you get a comprehensive breast exam by your Ob/Gyn or primary care provider, and possibly additional studies like a mammogram, ultrasound or MRI as they recommend. Of course also see your plastic surgeon for an evaluation of the implants.
Once this evaluation is complete and all the various issues relating to the breasts themselves, even things like hormonal changes, and nerve irritation, have been ruled out - then one can consider further consideration of the implants themselves. Assuming none of the evaluations above have established any defects of the implants, or rupture, it then becomes a process of elimination.
Once all these considerations relating to the breast tissue itself have been ruled out, and if patients still have pain, I offer my patients the option to retain their implants, or remove or downsize the implants (which loosens the capsule around the implant). I also stress simple things like wearing good breast support, as the weight of the breasts can cause pain, and even occasional use of medications like Advil if they find these helpful. Many patients still choose to keep the implants, depending on how much discomfort they have, and depending on how important it is to them to retain their breast size with implants. I have also removed the implants and substituted breast size to some degree with fat transfer.