I am sorry, you are being unreasonable and your life is far too intertwined with your DD's and DGD's.
Your daughter is a mature adult, in her late 30s. She should be left to lead her life as she wants to. She may have made mistakes in the past, it doesn't follow she is making a mistake now, and if she is, there is nothing you can do about it. You cannot live her life. What she does is nobodies business but her own and that applies to her daughter as well. I had a peripatetic childhood and went to 10 schools, I coped. I think your grandaughter will see living in Florida as a great adventure, settle quickly into school and be happy because her mother is and she is likely to have what sounds like a lovely step father.
I think you need to take a big breath and start to calm down - and then develop a life that is less wound round your DD, seek counselling and help.
For thoseof us who do not have our grandchildren on our doorsteps, whether a mere 200 miles away, like me, or many thousands of miles away on another continent like others. We manage to live fulfilling lives and love and are loved just as much by our grandchildren as they would if they lived around the corner.
Despite your rather hysterical post, I think at heart you are a sensible woman, so slow down, seek some help, learn to like the lovely American man who loves your daughter, and slowly, but surely start to cut the apron strings between you. Whether this relationship becomes permanent or not, that would be still be a good thing for you.
Anyway you do not know whether this will happen.If he has 18 months still to serve in the UK, they try a travelling relationship and then she does decide to go, that is probably three years away, and anything can happen in three years.
You are certainly getting into a quite unreasonable tizz, given the final stage of this drama is probably three years away.