Where to start..
Sure Weston is busy, last I heard they'd about 3 times the movements that Bal had, and more than Dublin too. But lots of traffic doesn't make for a complex environment. In Weston all the air & ground traffic is on one frequency, it's all VFR, and it all complies with standard procedures, most of the time.
Bal isn't busier, but it is more complicated, with 3 frequencies, mixed IFR/VFR, and non-standard procedures being the standard.
The IF Approach procedure to RWY23 passes directly over Dub at 10nm finals at 3000'. Believe me when I tell you, it causes confusion & panic, to such an extent that dublin simply don't allow it to commence.
Can't imagine why a 3000' TSA would be activated over dublin for a 500' AL03?? Unless it was a security operation similar to May Fly. The lads in the tower aren't a bunch of monkeys, there to make your life miserable, so there was probably a genuine reason to use 3000'.
Unless an aircraft is under radar vectors, it's up to the pilot to navigate around airspace boundaries. There is radar information available in Bal so it's very easy to tell if someone cuts a corner, and if one of our lads does it, he's just as wrong as anyone else.
Circling to land. The IAPs have at the bottom of the plate the OCA (obstacle clearance altitude) and on some, the COCA (Circling OCA). The COCA will differ depending on the Aircraft category A,B,C,or D, and there are restrictions on it's use. The pilot must not descend below that altitude unless he has become VMC. A 600'agl downwind is about 920'qnh and is perfectly fine. There's no extra danger associated with the procedure, because if the pilot is not visual by MAPt at COCA, he carries out a missed approach procedure. Easy as..
Just one last point. Glide & PFL circuits have a greater priority than 'real' circuits, for the same reason you say they don't. And by the way, ALL circuits are training devices. Glide + PFL are emergency training for 'in the event of a power failure after t/o and above 1000', or recovery from a power failrue at high altitude in a training area.