It makes it hard if you don't answer the questions asked - we are not there with you and cannot see first hand what is going on.

In any event, if the starter wont engage, I'd turn the key to "ON", wiggle the RCM out, and measure the voltage on pins E 5 and E 6 of the RCM. These are the engine control relay output pins. Pin 6 goes to the PTCM (which you are referring to as the "CPU"). If that pin does not have clean, reliable, 12 volt power, the PTCM will not be fully powered up.

So, my first move would be to verify that. If the pin is dead, you've got to fix THAT first.

If the Starter engages but the car won't start or wont keep running - I'd consider the Crank Sensor or the relay on the RCM that powers the fuel pump. (Of course, you can usually HEAR the pump run when you turn the key to "ON" if you LISTEN.)