R u sure negatives can replace pull ups?? i feel like negatives are more for those who want bigger arms ya?
You can't "replace" it. It's just something for beginners that can't do a pull-up yet.
Also, I see a lot of unintentionally bad advice here. The advices themselves are applicable to "most situations", but people here seem to forget/ignore that the nature of your problem is unconventional.
The advice they gave you is meant for removing bra fat caused by obesity or muscle underdevelopment. However, your bra fat, like you stated, is actually a DEFORMATION caused by a surgery, therefore your approach should be different.
Now the question is, how to get rid of that deformation?
Well, two things I can advise you to try. First is the same everyone else did: develop your latissimus dorsi (upper back) muscles. This might create pressure on the deformation, and "spread out" the fat better throughout your back. There is no need to lose any more weight, since I doubt that would influence the development of your problem.
Secondly, if you are really self-conscious about this, you can try liposuction. This, however, is not a cheap solution, but it is the only effective one I can recommend.
But, if you ask me, all of this in unnecessary. Like you said, you would have a perfect body without the bra fat. Are you sure about that? If I were a betting man, I would say that you would just find another thing to not like about yourself. I think it's your perception that needs to be corrected, not your body.

However you wish to proceed, I wish you all the best!