Not sure if its your car or you got bad information but my Corvette (2001) will NOT crank when the security system is activated. The Security light will blink when it does this and not crank at all. I hit the unlock button on my remote and it starts right up. Next time it does it hit the unlock button on the remote then try and start again.
If that does not pan out tracing it should be easy. Start at the starter and work your way back. When the key is turned is there a signal at the solenoid of the starter? If no then back track until you find the break.
A no crank issue is one of the easier things to fix. And no offense to your mechanic, but if he has been working on cars for 20+ years he probable knows more about Carbs then fuel injection. When I first became a auto-tech I specialized in port fuel injection. i.e. I know how to diagnose a car with electronic fuel injection, not rebuild a carb. Many shops would have me look at cars they already spent time with. Old timers could wrench circles around me but if it was not an easy fix or was newer generation they got confused fast. This may not be the case with your guy; he may go to all the clinics and stay current. But when I was a tech that was not the case for most older techs.
Also update your profile to where you live. Maybe someone here is near you and can help.