You shouldn't rely on the Park shift to hold the car it's not designed to hold the weight of the car, one day it will break and you will have damage to your gear box and what ever your car runs into. The P/shift is probably only a small 6 mm shaft that's drops into a hole in the g/box case. As for the hand break it should be able to hold the weight of the car forwards or reverse. If they are drum rears the shoes should automatically adjust as the shoes wear. If they aren't adjusting they are probably full of dust and clogged, and they are easy enough to dismantle clean and re-lube. If the drums have been machined and the shoes have wear you might not be able to adjust the cable enough. If they are disc's it could be the same that they have been machined down, wear on the pads, cable adjustment.