Are you hand or machine quilting it? If machine quilting will it be done on a sit down machine or on a frame system?
If the quilt will be hand quilted or quilted on a home sewing machine, where you will be basting the layers together first, you might have enough. You will be able to tell when you layer and baste.
If it is being done on a frame system where it is not basted first, then you will need more than 1/2 inch on the sides.
And yes, you can join pieces of batting together (but I would only do it if it will have a fair amount of quilting to hold everything together). I would just add it to one side, having the seam fall several inches in from the edge of the quilt.