Personally I have to confess I'd expect that with 2 cylinders 'out' it'd run like a lame donkey! Would be suprised to achieve run up RPM - I'd expect it to be painfully obvious something was wrong on takeoff. Notwithstanding the diagnosis, it sounds more like a fouled plug.
There's no reason that the thing should shake with one mag off either, on the occasion of an engine developing a somewhat unpleasant vibration after meeting a rain shower, I finished up completing the flight on just the right mag because it was misfiring and shaking when the left was on - there's two for a reason.
Consider also that I'm pretty sure your static RPM will be affected by density altitude (but so will the power output of the engine, so I'm not too sure of the exact relationship..)