crew are trained to look for the symptoms of possible intoxication. For legal reasons we do not suggest or accuse someone of either being drunk or intoxicated as this can only be determined by proper testing.
If you are however displaying signs of intoxication that cannot be explained by something else, then we will refuse service of alcohol as a precaution.
To be quite frank on an aircraft you are legally required to follow the Captains directions. For failing to follow a Captains lawful directions whether you like it or not you can be fined and jailed.
These laws are there for good and cogent reasons and often to protect idiots from themselves.
Rarely do we need to refuse alcohol but when we do the captains word is god on an aircraft. and the cabin crew act with (by law) the delegated authority of the Captain.
If a flight attendant allowed a passenger to conduct themselves in an unruly manner and other passengers complained it would be the crew whose jobs are on the line for breaking the law and allowing an intoxicated passenger to enter an aircraft or serving them further alcohol to the point of intoxication.
All responsible airlines observe responsible service of alcohol.
In terms of the issue of changing seats..... the captain must authorise all seat changes before take off. An aircraft is loading with freight and passengers (SLF) for weight and balance. Whilst it is unlilkey that a single passenger movement on a wide body aircraft would make significant difference to weight and balance, the rules are there for a reason and crew must be obeyed.
I would probably have let this issue go if the pax had moved back to his correct seat even if he had been a bit "mouthy" unless he was swearing etc
I dont take it personally when people get irritated by the rules as i can understand he might have been a little embarrassed as huge egos dont like to be told what to do and usually we are pretty good at massaging those egos.
what you will find however is that on budget carriers there is little place for passengers let alone larger egos and the crew are paid less than a tram conductor and really wont take any crap from anyone. As a jetstar flight attendant said to me recently for 35K a year I am going to do my best to look after them but at the end of the day, we are a budget carrier, if they want 5 star service dont catch a flying bus....