"HR is a big PROBLEM - no experience, no competence, wrong visas, questionable contracts, hold onto the foreign expert certificate as if they own it and take your passport constantly to have you tied down - some teachers sneak away in the middle of the night to avoid the fallout"
With the exception of the first two it seems that in large part these problems are the fault of the foreign teacher. A few minutes of research would have shown the foreign teacher that it is illegal to teach in China except on a Z visa. Then obtain the Z visa in your passport BEFORE you leave your home country and you are good to go. As for the "questionable contracts" again this should all have been taken care BEFORE you leave. Any questionable clauses should be either explained to your satisfaction or removed. If they refuse to do this start looking for another school. Regarding the foreign expert certificate why not sit down with your school and politely but firmly request it back? If the school is at all decent (something you should have been able to determine from your contact with them before you left) you should be able to hammer out a compromise of some kind. As for the passport refuse to let it out of your possession (with the exception of handing it to the immigration official to get the visa. Politely but firmly inform them that is illegal to give to anyone else. If they need to make a copy of it for some reason, go with them and as soon as the copy is finished take the passport back. If they say they need to keep it overnight, smile and say "sorry, that can't be done. My passport stays with me." Other than that refuse to discuss the issue with them. Once again the only person/people with the right to take your passport(for a limited period of time) are Immigration officials. I have followed these simple guidelines and while everything has not been perfect at my workplaces (who's are?) they have been more than satisfactory from my perspective.