Controlling NiMH charging with power MOSFET: MOSFET before or after the linear regulator?
It's not a good idea to do the switching after the regulator, since its COM pin will be left floating. The regulator might start to oscillate.

I would suggest putting the MOSFET after the 0.33u capacitor, before the regulator, and adding a 100n capacitor after the MOSFET. This way you won't have current spikes on each pulse to charge the big capacitor, but you'll still have something that filters high-frequency oscillations.

The 7805 won't "wear out". Being cycled every second is a very long interval, and I think it will be fine for the regulator. If you have shorter pulses (10ms or less), you might start having trouble.