Mandarin is very common in HK nowadays, and most people should be able to understand it to soem extent even if they don't speak it too well. Anyway, visitors from mainland China tend to spend a lot of money so it is to retailers' advantage if they can communicate.
No it is not, it is very common only where the mainland Chinese shoppers go to shop, shops in Causeway Bay, Tsim Sha Tsui, and Jordan, chain pharmacy like Watsons and Mannings, chain beauty shop like SaSa, jewelery shops, chain electronics store Broadway, some of the smaller pharmacy as well. (as mainlanders need to buy their baby formulas)
As a local, I can speak limited Mandarin, a lot more than I can actually understand what they said in Mandarin, not unless if they speak slow, but Mandarin speaking people generally speak really fast, so there is no way for me to understand what they said at all.
In the US, I actually speak English with the Mandarin speaking people, save the time to figure out how and what I should say in Mandarin, as using Cantonese to translate direct to Mandarin doesn't works at all, as we have a total separate spoken language, mostly full of slangs, Mandarin speaking people speaks what they write.