18 days certainly is a long time and as the email revealed the school had jumped through certain bureaucratic hoops to the extent to her a letter of invitation so I can see why they would be frustrated. That being said the tone of the email that they've sent is very unprofessional.

I too have recruited for a school in China and I know it takes a lot of work to separate the wheat from the chaff (and there is a lot of chaff out there!). But the work doesn't stop once you've found the right teacher. From my own experience, i can look back at the chain of emails sent to incoming teachers each of which is between 50-70 emails long! They'll ask all manner of questions about the school, academic content, the classes they'll teach, the area, the visa process, etc., etc. Accepting a job in a country 1000s of miles away after only having a skype interview can be quite a daunting prospect particularly if the school then isn't on the ball in replying to you about any queries you have. This is what Claire seems to allude to in the first part of her post and it would probably be just a useful to know what communication there was (or wasn't) between her and the school that prompted her to make the decision not to come after all.

Finally, Claire's decision not to go because she had doubts is far better than justing closing your eyes, crossing your fingers and jumping feet first into the minefield that is ESL. Most of the problems that teachers report on this board are probably the result of just that kind of thinking or lack there of. The school itself has admitted in the email that they think Claire accepted too quickly and without giving the position due consideration. But If that were the case then the school is as much to blame for the situation as Claire is - why rely on someone you believe to be unreliable in a given situation?