[Drisump]

I'm assuming that the questions posed neeed answering:
1. Yes the seal is the barrier than keeps the fluid from leaking out ....not the dust seal that you can see.
2. Seepage or age, they will all fail at some point, I rebuilt all my callipers last summer when one wheel was starting to drag.
3. The ATE seals sold by our host for our cars a a complete nightmare. They don't go on, they easily rip when you force the the issue...ask me how I know. After ripping one I went to a local parts place and they furnished a seal kit that was made in Germany ( name escapes me) that was easy.....WTF?
4. Deep pits on the fluid side of the seal will likely leak. If brake fluid is flushed every 2 years per Porsche, they shouldn't have pitting on the fluid side of the piston. There will likely be rust on the piston between the seal and the dust cap.....carefully polish out but unless extreme, no problem. BTW the bores of the calipers should also be nice and clean with hopefully minimal rust.

Cheers