Eeeekkkk, Sam, I feel like I've gotten so far behind from last Thursday
Ok, well first of all, I am so sorry about your situation with the dress from your Grandmother

I will keep my fingers crossed and say a prayer for you that something can be worked out. If you can't get it altered for you have you thought about incorporating it other ways? I would totally understand if you didn't want to take the dress apart but my friend did something that might work for your to still have the sentimental value to the wedding. She wanted to wear her mother's dress but it was like size 00000000

so (with her father's blessing) she used the fabric from the dress to make her bouquet wrap, her ring bearer pillow and the table overly for the table that had the remembrance candle that they lit in honor of her mother. Just a suggestion
Second, if you absolutely end up having to go with another dress I think you look stunning in your #1 choice (and you can tell you are completely comfortable in it) and my second favorite is the one that tied in the back that you think was a new dress in stock
Can't wait to read more updates