Wedding in 8 days, bust loose on dress

A bustier or longline bra may give you more to hold the gown in place especially if you can sew it into the dress. Maybe add with a silicone adhesive bra like found at Nordstrom and probably other stores. Alternatively (or in combination with other approaches), find a seamstress to help make everything work. And sewing things together would be better than safety pins, even if just a quick and dirty use of a needle and thread.

Some confirmation of part of this recommendation is that some strapless dresses have a separate stretch band or layer as part of the dress to help hold it in place.