The amount of retraction you need depends on the length of the Bowden tube. With no Bowden tube, 1mm is probably enough. Otherwise I find about 1% of the Bowden tube length + 1mm about right, using 1.75,mm PLA. I set the retraction speed high, typically 3600mm/min.
Its good that Tim is sending you a new board. I would triple check the wiring on the axis that the driver has blown on, especially look for any possibility that it could have an intermittent fault (for example if it moves or if something moves against it).
I'll second this one strongly. In the case of these drivers and most others an intermittent connection kills them faster then a dead short. The coils in the motors have some pretty nasty flyback voltages.