Re the dutch roll - it's been a very long time since I flew it, but....from memory, it is almost a non-event on the ATR and only done in the simulator to tick a box in the FAA-required syllabus. Recovery was to make a gentle aileron input towards the down-going wing, then when the aileron had 'caught-up' with the roll, make a gentle opposite correction to wings level. Then follow-up with gentle rudder input to re-centre the ball, retrim, re-engage yaw damper. Most simulators react badly to agressive rudder inputs anyway, so not a good idea to tramp on the pedals.
But, as I said a long time ago, so this may not be the current thinking.
RNP approval will depend on the individual installation. Some were built to an ETOPS standard. (90 minutes, I think?)
FL250 is a certification standard limit if you don't have drop-out oxygen and applies to several other turboprops, most of which are capable of flying higher, just not 'legal' to do so. The builders then tend to arrange the pressurization capability around this, so the cabin will be at or just below 8000ft at FL 250.
The AFM will state the maximum approved operational altitude.