Dribbling the ball as hard as possible in a stationary position does help with dribbling at full speed, however to get the best practice you will want to dribble the ball as hard as possible while running. I know I lose the ball the most when I'm running fast, looking for an open man.
The way I usually protect the ball is to turn to my side and dribble the ball alongside my leg so they can't reach low, making sure my elbow is in tight so they can't reach around and poke it out either. You always want to use your off hand to keep the defender away so they can't get too close to rip you. You can use it as much as you want as a shield as long as you don't completely shove the defender or slap their hand away. Though in streetball it doesn't matter, but it might get you into a scrap.