Theres nothing in RSA laws that say you have to prepare the drinks in front of customers, think of restaurants etc.
The most important parts of the law to watch in all jobs is
1. Service to under 18's
2. Entry and service to intoxicated persons
3. Secondary service of alcohol.
To answer your questions re drinks on aircraft...
1. Yes it is procedure to prepare J class mixers in the galley and Y class mixers in the galley except during the cart bar service,
2. The rules in relation to flying and RSA are the same as if we were on the ground.
If someone were to "top up" or drink their own beverage (which is common on intl services where theres lots of duty free on board) they are first told that they can't do it as they are on a licensed aircraft and asked to stop (a lot of people aren't aware its illegal). If they continue, it depends on the situation and how the crew choose to manage it. Generally the alcohol would be taken until the end of the flight and further action maybe taken at the choice of the CSM/Purser.
Its generally pretty easy to tell if people are topping up, you know what drinks you've served. Aircraft cabins are small and theres basically always a crew member watching the cabin, its noticed pretty quickly.