1. I think you need to ask for the forms. The mechanism by which they should find you is that if the dependant's business is more than a certain size (HK$25K in a year I think) then they should have registered that business, probably as a "sole proprietor". Once they do that they they will be on the IRD's radar. Strictly you are required to register within a short period of starting the business.
2. In principle I think not, but obviously you will need to have good documentation to demonstrate this. For stuff done partly in and partly out you can do some pro rating of taxes in some circumstances.
The IRD website has quite a lot of stuff on it, and [email protected] has always been helpful for me.
In my experience, if you are up front with IRD and clearly aren't hiding anything then you always seem to end up with the most lenient interpretation of the tax rules. But if they get the impression you are trying to hide something then they can become a lot more difficult. Given how low tax is here anyway then I reckon it's best always just to be fully open with them.