Please help

That's sounds pretty tricky.  If you had a single mastectomy, then you'd at least have one breast to help keep the dress and bra up, but not sure how well that works when you've had a bi-lat.  The support strips would probably not be strong enough to hold a form, bra and a dress up.  They still recommend using a bra with them, it just allows you to use any unpocketed bra. 

The corset idea might not be a bad one, as it won't be prone to sliding down, since your waistline will hold it up in place.  Also, you could see if the bride would mind if you wore a stole to help conceal any issues.  If the dresses were ordered before you knew you were having a mastectomy, then she should be willing to be flexible with your gown. 

Good luck!  Hopefully someone else will pipe in with some ideas for you. 

American Breast Cares has a strapless bra now.  Style 112...Maybe you can find a dealer that sells their bras!  Good luck!

Would you consider buying smaller, non-silicone inserts for the occasion? Maybe they could work with the strapless swimsuit, though with only six days left, it might be hard to find ones that would work just right.

Also, here's a link to a couple of bandeau bras. They might be worth a try, but I'm not sure you could get them soon enough--http://www.womanspersonalhealth.com/categories/364/1/Bandeau-Bras.htm

Edited to add: that does seem inconsiderate to choose a strapless design knowing your situation. I've seen weddings where everyone uses the same material for their dress but each woman wears a style that flatters them, some strapless, some not. Could you get a little extra dress material and have a seamstress fashion straps for you? I know you might feel a little self-conscious as the only woman with that style, but it might be better than worrying all night that your forms are going to fall out or drag the dress down. 

That was VERY inconsiderate for them to pick out strapless gowns knowing your situation.  What in the world was she/they thinking?  Sorry, I just had to say it!

Another thought - if the bride would go for it - would to be to make some sort of halter strap that could be attached to the center of the dress front or to either side and go around the neck to help hold it up.  You would make it with some sturdy cord with glass pearls strung on it.  It would depend on how the top of the dress is cut.  It could look quite pretty.