Bro, I guess there wont be any issue for going from mangla cantt. Its open for every one. I visited there from the path i shared with you. Dudial side will become way too long for you.
When you will about to enter the mangla resort. They will take your orignal Id card and will give you a parking sticker which will be placed infront of your car. You will definatly like the resort side. Its a marvelous place. Water view is amazing out there. They even offer you lunch in the water. But they demand 5000 for it.
If still you have any inquiry you can contact Saif ullah. He is i guess, haval daar (Army rank) there and is boating incharge.