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Hi
You do know that English is an official language in Hong Kong, don't you?

(and that standard Chinese is spoken at home by less than 5% of hongkongese...)

Some facts. According to basic law Hong Kong has two official languages, Chinese (spoken language, in reality Cantonese, written language traditional Chinese, not simplified) and English. How difficult it would be to conduct business in English depends on what kind of business we are talking about. In case you are working at corporate level, English should pose no problem at all but if you need to communicate with e.g. street hawkers or small shops or Chinese restaurants they often speak Cantonese only.
In terms of legal documents, such as legislation and business contracts my experience is that English takes presedence before Chinese, and many people, also Chinese, prefer it that way due to the lack of grammar in Chinese often making a western language like English more precise.
The long term trend is without no doubt that English loose in importance in daily life. The Hong Kong population is about 95% Chinese and they are becoming more and more due to the influx from the mainland, mainly Chinese from Guangdong with no or very limited knowledge in English.
Another very important aspect in the differences between the mainland and Hong Kong is the mindset. If you are used to do business in the west Hong Kong is easy but the mainland can be full of surprises.