I think Kevin can only teach magic to wizards (and only wizards), he can't cast any magic himself, except perhaps in the process of teaching his apprentice (in which case either he doesn't have enough power to harm the Golem or he knows Ashely isn't ready to learn such spells and isn't willing or able to bend the rules on when he can use magic).
As for the magpie, first of all we don't know it's going after the Golem - it could be going after a fourth, as yet unseen magical artifact.
Secondly, while all these other artifacts activating around it might be what triggered it to act, it's also possible that it had been intentionally shut down and the magic released by the destruction of the Dewitchery Diamond turned it back on.
In fact, I suspect that all these artifacts activating now is only related to the Magic Change in the sense that there are more potentially functional artifacts in the facility now than there were earlier in the day; I think it is the energy from the destruction of the diamond that's actually turning everything on. (Which means the same thing would have happened on a smaller scale even if Magus had done his resurrection trick before the Change. Pandora probably knew about this and didn't tell him, hoping to watch the fireworks; she'll be disappointed she missed it.)