I should have thought that walking/crawling to mid-wing would have overstressed the airframe (unless its been deliberately strengthened).
The wings provide lift while the fuselage is where most of the weight, and thus the gravity is.
Move weight from the fuselage to the wings and the stresses will become lower, instead of higher. That's why fuel (and in case of gliders: water ballast) is usually stored in the wings, and not in the fuselage.
So other than making sure that the top surface of the wing is strong enough to support a person, I don't see the need for any other strenghtening.)
Also, wouldn't the skydiver's body disrupt the airflow over the wing and cause the glider to enter into a spin? How did the pilot counter-act this?
You can't spin an aircraft unless you stall it. So as long as they keep the speed up they should not spin. The caption of the photo claims 100mph, which is plenty above stall speed (a quick Google shows about 30 knots for the stall speed). There will be yaw though due to the asymmetric drag, and an asymmetric loss of lift, but this is countered with ailerons and rudder.
You can see the differential aileron, by the way, in photo #2. There's no good shot showing the rudder though.
(I was going to say "simply countered" but I think this stunt is far from trivial, particularly in a glider.)