Bikes on Back?

Yes, you want things secure back there. Especially since you can't see what's going on while traveling.

I just double-checked the Elkmont's 2" bumper bar and unfortunately it's only welded onto the butt-end of each frame with 2 vertical 2" welds. No welds at the top. So I would definitely want it reinforced with a metal plate welded across the top from the frame to the bumper bar. Which would mean even more cutting and fitting of the bumper sheathing. Ugh.

The factory receiver may really be a lot easier and neater looking. And presumably it's mounted to the frame more solidly than the bumper bar is.