Oh Honey,
I feel you!! I tried on so many dresses. Eventually, I found one. I bought Casablanca #1900. I had them make changes to it to suit me, shorter train, lighter fabric. Some designers allow changes. Something to keep in mind. Good luck!!
I think asking for a donation to charity is great. I noticed on the honeymoon wishes registry that is an item listed. You could sign up for that registry and only list that one item.
Our engagement party is saturday, so I will post my favorite Sunday. So far we have received a few items from our honeymoon wishes registry and a cash gift.
What a tough situation. I think I would be direct and say that you need an answer. I would tell them that I understand their situation and will harbor no hard feelings if they are not up to it.