GF-2016: Is it possible for non-MS/AC nationals, to have left MS/EU after having spent 5 consecutive years doing research in MS/AC; so that they don’t have to be research active in MS/AC directly at the deadline time?

In the Guide for Applicant and in the H2020 Work Programme, Long-Term Residence means "a period of full-time research activity in the EU Member States or Horizon 2020 Associated Countries of at least 5 consecutive years".
We can confirm that for a Global fellowship, this criteria of Long-Term Residence is indeed a floating period of 5 years without time limit in the past. So in your example this Chinese national would be eligible.
(The same floating period approach for LTR would also apply for the Re-integration (RI) fellowship, which is designed for fellows who must move of have moved from a Third Country directly to the MS or AC where the beneficiary is located).