IT IS BEAUTIFUL! I agree with the hobbs Heirloom 80/20 batting- it is a very nice batt to work with- or even Dream Wool (which is my favorite of all time batt)
i would not use an invisible thread- i would use a nice verigated thread that goes from light greens into blues.
i would also quilt each section separately- but actually can visualize an all over working- maybe some echo quilting around some of the appliques- and some background designs (like clouds, or grass, or trees) in the more open areas---
my goodness it is just BEAUTIFUL!