To avoid grip locks and late releases, you need to work with your grip. I use a grip that is more “open” when I throw my P2s. Its close to the regular fan grip. I put my index finger on a rim and my middle along the bottom of the flight plate. My ring and little fingers are towards to the inner rim. Grip is very personal, but you should try this.
More important is the feel of the grip. First of all, when I´m throwing putters, I use stand still throw. It´s more accurate and you do not need to mind about your footwork. Typically, my putter throw is between 100 to 300 feet. When I throw my P2 putter, I use more thumb pressure. I form my grip by pressing my thumb towards my index and middle fingers. So, my grip is more tight on these finger tips than its in regular power grip where there is more pressure towards palm.
I hope you can get some tips from my style and you are able to build a confidence to your putter throws. They are really accurate when you master them.