I know the concern - there's nothing worse than a top that's tight across the shoulders, and even worse that there's not usually much you can do about it!
Since it sounds like it fits otherwise, and is only snug when you lift your arms, you might be able to get away with a spanx shirt that will help sleek things and make the top fabric slide more smoothly. (On the downside, sometimes the spandex shirts are constricting on their own - you may feel like you can't lift your arms because of it!) Here's the spanx brand version:
http://www.spanx.com/product/index.j...ductId=3069441 but there's others available (Hopefully this is an "acceptable" link - if not, I apologize!) Obviously, this only works if it's a long-sleeved top, but I would assume it's got some sort of sleeve or it wouldn't constrict this way.
Also, this may sound silly, but have you tried just wearing the dress (or just the jacket) around home for an hour or so? Sometimes even the most rigid lace has more "give" to it than we realize - it may loosen enough with wear to be comfortable. (But that's not something you want to wait until the wedding to find out!) That's the reason even the best-fitted strapless dress ends up being tugged up all night - they stretch slightly with wear!
Good luck! Best advice has already been given - eat clean, "real" foods, minimize chemicals and salt, and lots of water! Frankly, even if you weren't trying to lose and just wanted to look best, you'll be well off for eating well and drinking lots for a couple weeks before the wedding!