My .02 on the strapless....I dig the cardigan idea. Could you imagine if you sat down and that strapless dress just slid right on down?
The cardigan might give you a wee bit of protection.
(I just bought a dress at Wet Seal for a wedding. I was in such a hurry that I didn't look around. No clue that it was a hootchie store. Until I tried on the dress at home and my bum was hanging out.)