I'm just curious: in your sagas, what's the minimum disturbance a magical Circle (the one that a magus must draw to benefit from the appropriate Target/Duration) can take before being "broken", disrupting the spell? In particular, in the case of a circle engraved on the floor, would it be disrupted by someone simply stepping upon it (in some sense "interrupting" the circumference, but not really affecting the stone engraving) or would it need to be physically torn apart?
In my sagas, breaking an 'impromptu' circle can be done accidentally by interrupting the circumference in any way - stepping on it would be a border case, I think, in some cases it would be a disruption but in some cases not.
Formally prepared and engraved circles are not very difficult to disrupt for someone who knows what they're doing, but it can't be done accidentally - except of course by some sort of extreme violence to the circle's physical form.
No.
Yes. The advantage of a well prepared circle.
Drawing a chalk line across could be enough.
Maybe it would be worth it, to research a spell doing this (or something similar to break a circle target).
Since it would not have to penetrate Parma and could be done with a wide combination of arts - it could be a good alternative to dispelling magic with PerdoVim, in cases where it is applicable.
Should usually be doable with spontaneous magic
Although a ring duration muto spell to make the ring nearly indestructible is easy and will foil most attempts to physically break it. The right spell will bypass that I guess.