The cause of your pain may be Carpal tunnel syndrome. It is common after a colles fracture. in this syndrome, there is a compression of the median nerve due to an alteration in the anatomy. This can be corrected by a surgical release of the carpal tunnel. The best way to diagnose it, besides examination, is to do a NCV (nerve conduction velocity) test of the median nerve.
The other possibility is sympathetic dystrophy. This is due to alteration in the autonomic innervation of the limb, leading to significant pain, swelling and decreased range of motion.
The other possibility is a non union of the osteotomy. That means that the osteotomy done for correction of malunion has not united and that can give rise to significant pain.
As far as the range of motion is concerned, you should not expect increase in the range of motion, one year after the intervention. That is the maximum you could get, depending on the type of fracture, malunion and recorrection.