First of all, DONT PANIC! I know it probably doesnt feel this way at the moment, but everything will be fine!!
I think you should try the shop attendants advise first and add in a corset back, you'll be able to add a good few inches by doing that.
If that doesnt work then yes, get a back up dress. Worst case scenario.. You where the backup dress on the day and stick the smaller dress on Ebay or Stillwhite. 😊