There's nothing in the rules that says there is a "use" limit on landmines so to me that means there isn't one. They keep working until the ward is destroyed. Oftimes the base "reflect" ability of a ward is defence enough; the landmines are only for when it is breached. One point of complexity per "use" seems fair if you want to limit them, though.
No, but doesn't Harry worry during the Zombie assault whether they'll have enough juice to handle the horde? Or was he only worried about that many zombies just eventually breaking the ward anyway?
Either way, it implies a limitation to the potential of the ward. Perhaps the "landmines" might go off indefinitely...but any single one doesn't go off constantly, and they don't go off all at once. That implies a somewhat limited number, to me anyway.
Perhaps it works like this... The threshold determines the maximum potential of any wards anchored to it, and the ward determines the number and/or strength of anything like "landmines" that are attached to it. The specific numbers I'll leave for someone else to decide.