Let's work together Bron. I think that we can help you fix your issues. The use of Sanran Kagura characters is pretty extreme, considering their circumstances as being a close to H-series by themselves. So first of all, megalomaniac characters are actually pretty common. Have you considered more toned down characters like Gasai Yuno or Ayase Aragaki? Since your description only deals with appearance, their general characters may suit your needs as well. Don't worry about breast size as much, since we're going to help you fix that problem.

Large you-know-what happening quickly > terrible back problems > doesn't want to get a reduction > alternative solution provided by Aqua > goes to work for Aqua after graduating.
Well how about some sort of accident causing her to need the plating that supports her spine? I'd imagine training at a shinobi school would be quite dangerous, so maybe she was training at an obstacle course of some sort with live ammo, and she was running atop a roof when a stray bullet got her, causing her to fall directly on her back, cracking her spine. She chooses to undergo surgery to have the metal reinforcement, etc. etc.

I think you have a detailed back story there, and minor changes can change the uncomfortable issues others have been having. Why not, right?