Inevitable. Apple has too much of its money overseas, and doesn't want to pay the taxes to bring it into the USA. The only way to deal with the problem is to move its R&D to where its money is.
As far as avoiding leaks...my guess is the Chinese staff won't be told what they're working on or why. Design an IC with these specs, design a sensor with this range, lay out this board with these dimensions...don't ask what it goes into, or why. The less they know, the less they can leak.
Avoiding leaks is not the problem; it is blatant copying / theft of intellectual property. Chinese companies are already copying Apple designs, now Apple is giving them the blueprints. It will be much easier for a Chinese company to simply duplicate Apple's technology under their own name and there will be no recourse for Apple--we already know how justice is served in the Chinese court system.