You are allowed to leave the OMFG zone to go to the toilet/bar as you would any other seat at the venue.
I might be wrong (it's been a while since the McBusted tours) but I seem to remember that you get wristbands for OMFG when you arrive at the venue. And I think there's a separate queue so they can 'check you in'. So if you leave the OMFG zone for a wee/drink, you just flash the wristband at the stewards manning the two entry points to OMFG.
More people crowd round the stage catwalk and front barrier at first. However, with the boys running round the catwalk, you'll probably find that people will move freely round the zone. It's not a moshpit but it can get a tiny bit crazy at times. People are generally considerate though.
How busy it is, depends from venue to venue.
If you want to stay in one point, like you're set on being in front of Danny at the front barrier, the whole gig - get to the venue early. The further forward you are in the queue, the more chance you have of bagging a particular spot.
Hope you have a great time Johanna.