Most of the major's retail staff, at least one is fluent enough in English.

HK phone service has a different pricing model, you pay for the phone, and you pay for your contract usage. One is not subsidizing the other.

You can get any version(size) of sim card for any phone that uses it, including phones from the last millennium. Many of the domestic helpers/maids are using these older phones to call home. The only issue from older non-HK phones is whether the frequency/band is supported or available in HK.

Most places will have phone shops near each other so it is not hard to price compare.

Fortress and Broadway is the Bestbuy of HK where one can buy the phone handset.

There are some plans which require HK residency ID and/or HK CC account.