This does not in the least solve your present problems, but when bra straps showing isn't so much an issue, bras that clasp in front instead of in back can be really helpful for people with slim/narrow shoulders. The straps don't fall down! Instead of the straps being parallel to your arms, they angle in towards the claps between your boobs, so they don't slip off.

I am sorry for all your bra difficulties. :( That does sound like a really difficult environment to be a woman in, finding bras that are clearly *there* but that you can't actually see - BLEAH.