I'm another in the goal camp I think. This one is a prime example of the need to piece together the components of both sections of the LOTG as it is currently written (hence the forthcoming rewrite).
So, Law 14 (p45) give the procedure and for me the crucial element on timing is that the first bullet point says:
- After the players have taken positions in accordance with this Law, the referee signals for the penalty kick to be taken
Therefore, the referee must only confirm that the correct positions are adhered to at the point at which the signal is given for the kick to be taken. After the point of the signal, if the GK turns before the kick is taken then it counts as a defensive infringement and as the ball enters the goal the goal should be awarded (as per p46 & p133).
Only if the GK turns before the signal does it make the kick invalid and thus require a retake.