O bhaio!! Have some sense!
When an auto is put in D or R, it keeps trying to move the car in that direction. So while backing down a slope in reverse, the engine is ACTUALLY TRYING to reverse - naturally the brakes will be less effective because they'll have to work against a reverse slope as well as the engine trying to reverse the car back down that slope.
When on the same slope, and in D (forward), the slope tries to push the car down but here the brakes get some help as the engine tries to keep the car going forwards.
So...its simple stuff. Its normal for an auto.
Rest, just keep your engine perfectly tuned up. Idle rpm means the engine speed while engine is started but you're not pushing on the accelerator. In neutral it should be around 750rpm, when you put car in D or R then it should rise slightly to 900rpm. Since these values are not written on the meter, you will have to judge by the position of the needle - how far it is between the 1 and 0 value.