You pretty much explained it yourself.
First check all grounds. Than go to the ignition switch. Then go to the starter solenoid. Dont know about that spray, but most anti-teft devices measure resistance in the key chip and that should not really affect anything.
It really sounds like the ignition switch. And what do you call a starter relay. No such thing. Most cars have an ignition switch that does both and the rest is the solenoid's work. What you could do to rule out the ignition switch is to try to hot wire the solenoid and see if it turns over.(maybe use some remote starter thing) You should figure how to do it yourself. Or just see if there is current in there.