So deeply sorry about DH's health issues, Lilylavendar. IMO, everyone should just be happy he can go to the wedding, and nothing else should matter.
I understand FDIL's concern about the dentures though. It may not just be about the photos, she might find it embarrassing to have a family member come toothless to the wedding. Petty, perhaps, but that may be how she feels. I'm glad he has decided to try to get used to them. In time, it will be to his own benefit.
It sounds as if her approach was insensitive, however, especially about the weight issue. But since the men are being measured soon, IMO, it will become a moot point.
As for your uninvited DD, I am so very sorry. I know it must be hard for you, as her mum, to see her left out. How torn you must be between the desire to attend DS' wedding as well as your appreciation of your other DD's being chosen as a bridesmaid and that one DD being pushed away. However, the guest list is DS and DIL's business, just as how DH handles his weight is his business. As hard as it may be, I think you need to accept their decision about their guest list and not expect them to invite all the siblings, just b/c you and DH do.
Perhaps you can enjoy a special day w/ the uninvited DD sometime soon, just you and her, or you, DH and her. Beyond that, I hope the wedding is beautiful and that everyone has a good time.