Have a look at
http://www.emb.gov.hk
1.) Certificate of Service. I don't have one. My last (and only) teaching job screwed me over and I left. Don't think the director thinks he owes me any favors. I've heard the CS is mainly for calculating your pay, is that true?
Yes, it's true that the CS is needed for calculating salary. Without the CS they might give you the lowest salary.
2.) What would be the salary cap (housing/baggage allowance included) for a Category IV (no teaching license) be?
As far as I know, the special allowance is 12950 per month. That's to cover rent and living expense. It's seperate from salary.
3.) How long do interviews usually last, and what are they like?
About 45 mins to 1 hr. They do ask you about your past experience in teaching and what strategies would you employ in teaching and teaching techniques. If I'm not wrong, there's also a test at the end of the interview. A written composition to introduce yourself.
4.) If a school's interested in me would I interview with them too?
After the interview and if you've passed the interview with EMB, they'll then pass your information to the schools and if the schools are interested in you, they'll call you for a phone interview.
5.) Since September is fast approaching I wonder what kind of schools will still be looking. Bottom of the barrel? Can I say "no" to a school and hope for another offer?
This question, I can't really answer. I'm starting in Sept myself and I took the first school that interviewed me. Before the school called me for an interview, I did some checking and called a friend in HK to check about the school. And obviously I did like the school and didn't mind working with them when they called for an interview. After the phone interview i had to wait a couple of days or weeks before they comfirmed me.
Hope I answered your questions.