I can help you with a few of your questions:

- Applying for an employment visa whilst you are overseas is recommended, if you do apply when you are in Hong Kong the immigration department will then ask why did you not apply before you arrived.

- You are correct that with any application for an employment visa the immigration department will look at the details to see that you have skills that cannot be found by the company who wants to employ you in Hong Kong.

- If the application is submitted with all of the requested documents by the immigration department it usually takes around 4-6 weeks.

I hope this helps.