Several (4) questions here Crab.
For HEMs approved operators:
Q1. Elevated (rooftop): yes. Non elevated; no. (company approved)
Q2. a, Air ambulance flights (CAT); yes.
b, HEMs flights; elevated, yes. Non elevated, whenever possible.
Q4. There are several. (Each operator will have their own)
Q4. Sometimes immediately, when out of the operators usual area (for HEMs flights only) (also perhaps used by an operator, in slow time, for one off company site approval, when preplanning a CAT flight out of area.)
But if a listed mil site is also one of the local operators HLS, cleared by that company for CAT, then that operator could use it immediately for CAT flights, without waiting for company approval)
Hope this helps! BT