Put the "firing LED" in parallel with the firing switch with a resistor large enough to drop the voltage below that which makes the igniter burn. When it's armed and you have continuity along the entire line (line is good, and igniter is hooked up right), the LED will light up. When you hit the fire button, it bypasses the LED and drops the full pack voltage down the line.
On your schematic, move the LED connection R1 to before the switch S2.
And get yourself a really, really cool set of switches/buttons for it.