Hey there,
Well here's what I think... for what it's worth lol!
1. Actually yes you will need to get assessed my the HKQAA, and it it expensive hey. But don't go sending your originals over, you can send them in, and when you arrive, go in person to present your originals and collect the assessment. Don't ever let go of your originals!!!
2. Yes, most International schools are private. Depending on the school, you might have to register with the EDB as a teacher so you will need the assessment. However, some schools will take you on the strength of your papers. I have found that a lot of places, for teachers, ask for the assessment report... useful to have in any case.
3. If you don't register with the EDB i'm sure you can still work in a million places! Learning centres, private schools etc. I only had to get assessed because I qualified and wanted to register as a teacher.
4. Yep EDB is the Education Dept...
5. I thought ESF was private, but actually I think they might receive govt funding? Not sure on that one
6. If you are employed by the EDB it is most likely you will be sent as an English teacher to a local Chinese School.
You should check out TES - Teaching jobs, resources & ideas from the Times Educational Supplement which has some great jobs, and perhaps more the sort of thing you are looking for. Go to the international jobs section and check it out....
Plus if you apply for a job overseas, the company will sort out all your visa paper work etc. I never sent off any originals with my applications, I was never asked to... so be really careful!
Goodluck... i'm off to have a cup of tea after writing all that! I'm not sure if i'm right though... but that's what I seem to have in my mind!
See you for a few drinks when you get here!