There is another aspect of this that no-one but Jedi has commented on, and then only briefly:
"I have a problem with this DPE acting like a DME (Designated Mechanic Examiner) and forcing the applicant to accept the DPE's incorrect information."
Folks, the examiner is wrong. It is entirely possible for the second guy to receive a second STC for shortening the wings of his airplane. The FAA won't issue an STC for the exact same modification, performed in exactly the same manner, but if the second guy came up with a slightly different modification; say cut off only 2 inches, instead of the 3 inches that the first guy cut off, or he used a different wingtip after shortening the wing, he could get his own STC (he would have to, as his modification wouldn't comply with the first STC.
Just for example; there are at least 2 different STC's held by 2 different entities for installing an O-360 lycoming in a Cessna 170, there may be more.
Another example is STOL kits which have leading edge cuffs and stall fences for 100 series Cessnas. Just off the top of my head there's Horton, Owl, Bush, Sportsman, and Robertson (although the Robertson conversion also makes the ailerons droop as flaps are extended), but they all involve leading edge cuffs and stall fences at the Aileron/flap junction
Anyway, saying that the second guy would have to buy the STC from the first guy if he wanted to shorten his wings just ain't true.