I'm going to make a few changes/notes here.
Why does the field still tear apart magical circuits and the like as though it still had the absorption properties? Making it more difficult to cast and making some stuff not work so well makes sense but the offensive destruction part does not fit in disruption. Ripping apart circuits is a very specific type of origin anyway. Ie like Origin bullet. In exchange, so that she actually does have an offensive ability, I propose giving her a concentrated field, one meant to mess with a single target to be used at a much closer range. The smaller the field, the greater the disruption. So, for example, using it on just a magus' head would have pretty great effect, making incapacitation a pretty doable thing. That would actually mess with the prana flow in their body, making it go haywire like a much more temporary and weak version of Origin Bullet. It would not wreck magical circuits inherently but you could put it like making a human's blood flow backwards in random places. There would be internal damage and casting through it would be nigh impossible. Of course, it would be a bit harder to land too but Kana isn't so trained and that is but one facet of her origin, not the entire thing.
Just like with the magus, prana shaving from a servant makes little sense with them being only more resilient and shaving having little to do with disruption anyway. The anti-servant qualities for rank downs are fine but should be applied to all servants. There is no cognitive reason that Kana's should be exempt. Fields can be considered akin to rule magic. The rules are set in place and unless you shatter the whole thing, everyone plays by those rules. Caster was exempt from the Ryuudou field for the same reason that everyone could have been. Going through the main gate was a hole in the barrier due to the temple theology. It just so happened that she covered herself fairly well by placing Assassin at the gate to prevent others from getting in without getting their power cut down. So unless there is a wide spread rule of sorts like "Being within 3 feet of Kana makes you exempt" or "Touching Kana makes you exempt" or whatever, her servant would be cut down in power too. There are no free rides. Having a connection with Kana would be far too specific and nonsensical a rule because the connection between them would naturally be included in the effective area. The field operating in such a way to not hurt herself directly makes sense but protecting any of her magic other than the spell or whatever beyond the basics of keeping the caster herself safe would result in the field itself creating numerous random and complicated dead zones. Keep in mind that disruption is the primary job of the field, not draining like Shinji's crap field or dampening like the Ryuudou temple barrier. Servant weakening is something that I suppose could certainly be trained to be included but it would never be as strong as a dedicated one like Ryuudou and isn't part of Kana's origin either.
Phantom is correct on the other counts.
