Insert a waypoint and change it to DO_SET_SPEED. It is in MPH. The default waypoint nav speed parameter in Arducopter is 10 meters per second, which is about 22mph. That is what it will use unless you put in a DO_SET_SPEED command as part of the mission.
The yaw behavior when it comes setting or clearing an ROI is not an adjustable parameter. The turtle/rabbit camera pan slider will probably effect that. But there is no making it logarithmic move in and out of that like it does with smart shots.