I find myself rather in agreement with Squid, on this one. Each of those points raises a chuckle.
Diplomas in TESOL are certainly considered better than Certificate courses, but they typically require students to have already attained a Certificate and have some considerable teaching experience. Such, reputable diploma courses can take up to twelve months to complete. Only then are they taken seriously by (reputable) employers.
The guaranteed placement angle is not a winner for teaching in China, and may even set you up for a lesser job than you could get on your own merits. Check out TEFL.com, which vets employers before allowing them to advertise jobs. Also Dave's ESL cafe. There are no shortage of TEFL jobs in China and you can even land some pretty decent ones without any TEFL qualifications at all. Furthermore, if it seems as though you are being fed into a particular school, then you should be extra wary about the nature of the relationship between the centre where you qualified and the institution where you are being sent to teach. Since anyone who has got that far has already got a track record of having the wool pulled over his or her eyes, you now run the risk of setting yourself up for a year or two of under-paid work, with the added possibility of dubious legality thrown in for good measure. Dancing banana time.
Learn so much in a month? Well, yes, you can. Intensive certificate courses achieve this all the time. Diploma courses don't. At this point your post reads more and more as though you have, unfortunately, been suckered in to buying one of those oh-so-cheap 'iPhones' from Shenzhen...
And the job in the mainland that isn't exactly the job that you wanted? Well, after all that you wrote up to that point, this, really, was only to be expected. At least you have maintained a possitive outlook and, as you say, the experience you gain will no doubt stand you in good stead, later. Be aware, though, that employers who care about qualifications, whether in mainland China or in Hong Kong, will likely not care much for your diploma. If you do end up in Hong Kong, therefore, it might well be in another job that is not exactly what you wanted.