First of all, what do you mean that your med school limits you to one away? My school only gave us credit hours for one away in ortho (or any specialty), but we could do as many months as we wanted if we took less vacation time. Our total requirements for 4th year would have allowed for 2-3 months of vacation, so it was plenty easy to fit in an extra month of ortho if you really want to.
As far as a late grade, if you do end up only doing one away, I doubt the determination of whether or not you will be granted an interview will be based primarily on your away rotation grade. There is a lot of other stuff in your application that will determine your interview offers and you can submit your grade from your away as soon as you have it.