I'd add to the batting. I know I wouldn't be happy with the end result if I didn't, especially after taking time to put the top together.
I agree. You always want to do your best so that you can be proud of the work you have put into a quilt. Do you have extra on another side? I would only use the same brand of batting. I always use extra stash to help if I am short on one side. I cut the edges so they are nice and straight, butt the edges together and use strips of lightweight fusible to attach so that no one will know except you and us. :)
