Bra and lots of safety pins..

I think that would look HOT on you. IMHO, it's fine to have the center strap exposed if it is the same color. I don't think it's ideal, as far as the hotness goes, but I don't think it looks bad by any means.

Somebody makes a bra with a clear plastic center strap to avoid that issue. . .I had a magazine that mentioned it. . . I'll see if I can find it.

take the dress to a bra shop and try them on together to make sure they work.

once you find a bra that works you won't ever have to keep fussing with your outfit or worrying that you are showing more bra than you should. and it will probably help with other outfits too.