I agree the geography is crazy, but I've been allowed such stopovers before. (ex: BOS-TPE (destination), TPE-HKG(stopover), HKG-BOS). The agents have always consulted a list of "valid stopovers" before confirming the ability to do a specific stopover. As long as the S. Asia zone and N. Asia zone require the same number of miles, there are no additional miles required for the stopover.

I think it's the routing causing the problem--call UA back, ask about doing a stopover, and maybe you'll at least get an explanation.