I think that once the ward is breach it does NOT go down. In grave peril when the zombies keep coming at Harry's apartment they keep blowing up until they slowly weaken the ward until it collapses.
That would make me think that the "wardmine" would keep going off whenever the ward is breached no matter where the extra shifts are going. But when the ward is nullified the "wardmine" would go away with it.
I would think that you could also set a ward to physically attack anything that touched it or damaged it. Seems to me that in the example of dresden's wards and the zombies that none of those attacks "Breached the ward" because then by the above rules they could have moved past said wards (if an attack can go through once the ward is breached I would assume anything else could). That would in theory have charges but really I have no rules backing me up on this, just a sense that that could be a way for it to work.
There are several things going on, if I understand correctly:
1) the Wards are reflecting back any attacks which don't breach it, hurting anyone who touches or casts spells at it (so the Zombies are taking damage from their own attacks
)
2) the people attacking the Wards are instead opting to tear down the Ward with the extra shifts they are generating (which is why the Zombies eventually tore it down
)
3) even when you opt to tear down the Ward itself, it may still be considered a breach, which means a Wardmine in the face
Do I have it right, folks?