Keep things in perspective - it is never wrong to give a demo class when asked for it,

Okay, according to you, a foreigner in China 16 hours by train away from a prospective Chinese employer should waste much time and money to give a demo class, in the knowledge that there's no guarantee of a job offer, and that unqualified Chinese assessors and students will be the judges.
You can say the foreigner who agrees to do so is like a very stupid duckling that strays within biting range of a crocodile, without knowing whether the reptile is hungry or not!
By the way, one of China's most prestigious universities had recently invited foreigners, even those residing abroad, to give demos at that esteemed seat of learning without any recompense for travelling expenses! Ridiculous, but ok re what you have stated!

but it's always wrong to work for a crappy training centre, especially when asking their teachers for a deposit!

Is wrong the right word? Do you mean illegal, I don't think so. Maybe we should replace 'wrong' with ill-advised!

I think you realise by this time that I'm not a STUPID duckling!