Don't quote me ... I'm no expert!
But I wouldn't advise you to have it repaired at the Pioneer service center you mentioned because I doubt their competence, considering that they weren't even able to diagnose the problem correctly.
Just tell the shop around the corner that the player won't read dual layers, but reads single layers just fine.
Bring along single layer discs, 1 orig and 1 fake; and dual layer discs, 1 orig and 1 fake (if you have one); and demo them all for the technician. The demo will eliminate a region lock misdiagnosis. He might ask you to leave a dual layer disc in the shop for testing purposes.