Using brakes to stop yourself while in reverse taxi can very easily set you on your tailskid. It depends on how fast you're going, your loading (aft cg) and how hard the brakes are applied.
The procedure to stop reverse taxiing is to stow the reverser mechanism. And yes, the aircraft will more than likely start to move forward at idle thrust. Again, depending on loading, pavement integrity (hot asphalt can cause an aircraft to "settle in"), individual aircraft type.
Modern turboprops are rarely operated at Idle on the ground. They have so much power that it is almost possible to take off with it. Now before the guns come out, I used to demonstrate why power MUST be at idle for landing the Dash 8: with a lightly loaded airplane and a long backtrack (back taxi in some books), I would leave the power levers at flight idle and watch the indicated airspeed climb to nearly takeoff speed. Not an indelicate exercise, but demonstrated when needed!
Although I stated earlier that reverse taxiing of jets is prohibited at a great many airports, obviously it is not prohibited at all! I've seen DC-9 type a/c reverse taxi at many Canadian airports, some US airports and a very few European.
On the topic of "how fast" do 757's or other large a/c backup? No faster than the marshaller can walk. Backing up without marshallers is like unprotected you-know-what: you might get away with it some of the time, but...