The Hatta border run will work until you reach a total of 90 days then the renewal has to be done at the airport which means a flight out. 9 flights a day to Doha so easy to fly there and return on the same flight. You can then do the renew at Hatta again until 90 days, then fly out etc. It varies depending on the passport your girlfriend has. As for living together lots of people do it just keep below the radar...