Your X1 complementary NPN/PNP voltage follower has the bases floating when the button isn't pressed, they could pickup noise.
The button current is very low and may not be enough to make good contact with time and corrosion, many precious metal contacts have some minimum operating current.
The zener D4 will almost certainly be horribly leaky at those micro-amp currents and not work the way you expect.
Here is one way of doing something similar, using a NPN/PNP latch triggered by a short button press but reset on an extended button press. The standby current is zero. The button current can be many milliamps to tolerate dirty or worn contacts. I separated the latch from the PMOS load switch but you might be able to combine and eliminate the PNP - I wasn't sure if the load had a lot of capacitance etc.
piglet