ESL teaching jobs at training centres and certain other Chinese schools are not quality jobs an such that are at the bottom of the barrel.

Agreed.

Thus, only folks unemployable at home - whether because of lack of education, personality problems, or of a record of crime which causes them to escape from justice back home - would take such jobs just in order to survive.

Disagree; replace 'only' by 'some', and I'd agree. You should understand that westerners back home unemployed, not necessarily unemployable, and others in deadend low-paid jobs are lured as FTs to China by training center adverts offering high pay and career structures of various kinds.

Such folks for SURE will become mentally ill.

Disagree; most won't, but some will - especially the older westerners among them. Case in point, the old man with little to do except post loads of useless drivel mainly on the discussion forum, so annoying most readers of thr board, including you and me!