The question is, why should GKN and CFM/GE make a loss for the Chinese? The Chinese like to play by their own rules, including schedules. Why should CFM/GE employ a large number of individuals in an idle factory? If China wants the engine, they'll still buy them.
There is no business case, until the C919 is in production, for another engine factory. Coupled with the ongoing concern over IP theft (there are many closely guarded secrets in the aviation industry, especially around engines), companies are voting with their feet. If Pratt and Rolls Royce both refuse to do the same, what choice with the Chinese have if they want any kind of credibility? The Chinese aren't in the position to be making demands here, and if they can't lure these big three OEM's to China on the basis of lower production costs, they only have themselves to blame for past behaviour and business practices. This is when your past behaviour bites you in the rear.
Also, it's not just COMAC, it's the entire regulatory process in China that's broken as shown by the ARJ-21. Neither COMAC or the CAAC know how to certify an aircraft; the engineering justification, flight testing, and data analysis is a very complicated process, which in many respects is actually considerably more complicated than the actual design process. It's gotten to the point where the CAAC has pulled in outside experts to assist them from the US FAA to help the process. You also can't do certification process on the cheap for a new design of a large jet; Airbus for example will invest in and will be running 6 telemetry rooms concurrently for the A350 certification. Boeing has their own massive telemetry room and a van that they used for the 787 and 747-8 certification process, and also has a large backup facility if multiple tests and aircraft are being conducted at the same time.