Switch off circuit is too sensitive and inaccurate

C6 should be discharged through additional resistor connected in parallel, not only through R11 and T1 (because base-emitor junction of T1 is dependent on temperature and also T1's junction is not linear load for C6).
Switch off circuit should use lower impedance signals, to be less susceptible to interference, and to supply enough current for zener diode to keep voltage regulated.