First, it is possible to hop back thru the invisible wall. Just Ghandi-hop ( crouch-jump + crouch again) onto the small ledge on the wall of the level, and jump right past the concrete wall back on to the roof.

Second, it is possible to get past the invisible wall without a "dummy-guest". Just have your friend who already got thru stand in the same corner on his side and jump. The guy who is still on the roof side sword-cancels him and squeezes thru the crack no dummies required.

Third, you can get back up from the beach-side OOB territory. If you both are down, butterfly or sword cancel. If one guy's down and the other is up, the guy who's down finds the highest point on the giant rock and just sword cancels the guy up top.

seraphimwarriorx