I am a mitralvalve repair survivor :) so I know a little about MVP by now. Given your age and the fact that you are asymptomatic means that your MVP is under control.
Good on you for wanting to be active and healthy :)
I don't think you should worry and focus on the heart rate so much. You need to pay attention to the symptoms you get when excercising! if you become short of breath or very dizzy, then slow down. If you get chest pains, then STOP and take it much easier.
Beta blockers will keep your heart rate down, but it also makes your bloodpressure drop and then you'll be dizzy or if you have high BP, then it will stabilize it.
Have you been for a stress ECG lately? that is the best way to see how well your heart copes under stress, i.e. excercise.
The only way I know of is through medication such as beta blockers which you are taking.
Is it right to assume blood pressure would be more of a concern than heart rate? within reason of course.