ok answer,
i have noticed to reverse uphill you need commitment
as you state as soon as you start pressing the gas pedal the brake releases, if the revs arnt high enough it will roll down hill
option 1 ( if nothing behind you ) select reverse forget the parking brake and hit the gas to about halfway it might shoot uphill a bit quick so be prepared.
option 2 ( my personal fav) dual foot control foot on the brake and on the gas, power up feel the bight and release the foot brake, the parking brake take care of itself.
problem with the modern autos is the use of clutches instead of torque converter that could hold the car on a slope, moden ones just disengage hence the roll