You absolutely CAN NOT use the same battery for your receiver as you do for your ignition. I am not sure if this is your intention, but just wanted to make that clear.
Now, as for the switch, you are making things WAY too difficult. Get a fiber-optic kill switch and be done with it. It is a switch in and of itself so you don't need an external switch to turn on by hand. It uses a beam of light sent to the switching unit so there are no electrical connections between the receiver and ignition switch. It doesn't require a servo to operate - just a free channel on your receiver that you can program a toggle switch or button on you transmitter for. And, in the event of a power loss to your receiver, the ignition shuts off automatically so you don't hit the ground full blast. It also has an LED that you can put wherever you want so you can see it and know when it is on. And the entire unit only weighs one ounce while what you are talking about would be a quarter-pound or more!
The one I use is the Smart-fly optical ignition cutoff (
http://www.smart-fly.com/Products/Ignition/ignition.htm ). I actually have a spare in my shop that I was going to put in the swap shop for $40. Let me know if you are interested.