The vi you provided does not include the SubVi's you are using so it is difficult to see exactly what is going on. The way you are handling the button presses has worked for me in the past. Though, there is another method that uses shift registers and limits the number of loops you have to deal with.
Usually when the NXT gets bricked it doesn't have anything to do with your code. Instead the cause is static discharge. Have you reviewed the FTC Guide to Addressing NXT Lockups? We had all sorts of problems with lockups last year but by purchasing a VERY short Samantha to NXT cable and adding a surge suppressor, we have had NO issues this year. With the exception of when we run the robot teathered using a long USB cable.
Hope this helps.
