Spocla,
Sorry for the repetition - problem with the "net nanny" making me believe that the message had not gone! I agree with what you say on the Dutch roll. It is an oscillatory dynamic stability mode that is damped by Nr (not rotor RPM but yawing moment due to yaw rate!), hence why a yaw damper feeds in rudder to oppose yaw rate. Roll damping (Lp) has a much smaller effect on Dutch roll damping.
JF,
This sounds like a divergent spiral mode which, as it is an aperiodic mode, is defined by the time to double amplitude. Therefore, the greater the bank angle, the greater will be the open loop roll rate and thus closed loop more aileron will be needed to counter it at 10 deg AoB than at 5 deg. Which aircraft did you mean? We are all now intrigued!