Foreigner can buy iPhone in HK?

You would need to go to the shop itself; you don't need an appointment.

But remember, Iphone warranties and services are only carried out in the country the phone was originally purchased from. As Apple do not ship across borders you will need to consider how you would achieve this were it to become necessary.

Buy in your home country would be my recommendation.

Right. Apple care = is regarded as an extension of the original warranty'. If that is not valid, then neither would be Apple care.

Apple have many stores in Japan. The people in the stores will examine a phone wherever it came from but they probably would not have the parts for phones from other countries, many of which differ slightly in construction.

So minor matters would probably not be ignored, but I mean, minor matters.