I already have a 10-year multiple entry visa to China. The max duration of each stay is 90 days. Would I be allowed to stay for an entire year, as long as I leave the country and re-enter at least once every 90 days?
Yes, you are allowed to stay as long as you have an exit and entry out of the China every 90 days. 10-year multiple entry visa's do not have limitations on the number of entry and exit. You can leave for 1 day and come back without a problem.