For the min IFR speed, have a read through the Advisory Circular for part 29 certification. There are many test points that have to be demonstrated. The fact that the min IFR speed is 60KIAS indicates all those points were demonstrated at 60KIAS and above. It would be dangerous to automatically assume that any of them failed below 60KIAS, but as Nick and Shawn have pointed out, there are some things that will fail at some point.
For reasons not associated with certification, I've done some of the certification tests on the 412 but not enough to offer a final answer. From that experience, I'd say that the weak longitudinal static stability at min speed in a climb is the test point with the highest min speed.
With one AP disengaged, the remaining channel has to double its gain. The series actuator moves twice as much distance for the same correction, in order to compensate for the lost system. Because of the extra distance it travels, it is easier to saturate the actuator. If this happens, it will feel like you have lost both APs in the one channel, until the actuator is driven back into range. If you keep your speed below 115KIAS (and fly normally) you shouldn't be able to saturate the actuator. With both failed, you don't run the risk of saturating anything as neither actuator is moving, so there is no limit with both APs disengaged.
Matthew.