How can I pull an op-amp output low without transients?
If you go for shunting R5 down to ground at least the opamp remains in control and not saturated. Then add a load resistor to the output of the opamp to ground so that it does pull all the way down to within 50mV of ground, the leakage problem you mentioned should disappear. Let me know if you try this out and it's still not effective.

The load resistor will be about 1 kohm and can always be in circuit ie it wouldn't affect normal operation.