How does a 120VAC neon indicator lamp respond to a leading-edge triac dimmer?
You can leave the neon in place, it will neither confuse the triac, nor alter your calculations in any way. When the oven heater is on the neon will glow. 

The neon is not like an incandescent in any way, it is a gas plasma (near enough) and is essentially open circuit when below it's trigger point. Your circuit is slightly wrong in that the neon will have a series resistor to limit it's current after striking. The resistor is likely in the 100k ohm plus range.