The "tourist route" on Lochnagar is ok as far as I can remember, Mar Lodge is NTS so no problems there.
As for the getting to know in advance, this can change from day to day.
It can be quite restrictive but mostly I've had co-operation from stalkers. I've told them where I've wanted to go & nine times out of ten been offered a route which doesn't cause any conflict. The only time I've had a point blank refusal was in Glen Ey, when I was told not to got there at all. I've also found it pays to speak personally with the head stalker, if you can get a number. Above all, be polite.
Remember, no stalking on Sundays so if you really are having difficulties use NTS through the week & go where you want on Sunday.