2. If you go back to China next summer, then you have to stay at China for two years to fullfill the two-year homeland service requirement. For sure.
3. If you go back to China after the master program, and aplly J-1 again in China for Ph D, you might get the visa but then you need four years homeland serive. If you hvae to apply waiver, then you have to apply two fo them. That is too much.
4. It possible way to deal with your issue is: Try to apply Ph. D. ASAP, and then as long as you get the admission from PhD program, start to apply waiver. If your original workplace in China is cooperative, it should not be too tought. But please remember that your admission for PhD should be F-1.