Zener Regulator supply to IC
Using a zener diode as a voltage dropper is unsafe and inefficient — at best you'll end up dissipating heat in both the diode and limiting resistor. As the load current is low enough you could (and should) use a voltage regulator such as the good old LM7805 in a TO220 package. For a 5mA load current the regulator would dissipate 50mW — i.e. \$ (15V - 5V) \times 0.005A = 50mW \$ — which is way better than any zener + diode contraption. As such there's no more need to hack your circuitry, much simpler, much safer.