Quote Originally Posted by alexjb1989 View Post
... I have put a multimeter on the battery and it reads 12.4V and does not drop below 10V when cranking
The battery is fine.

Perhaps what you mentioned about the immobilizer is correct, and it needs to be reset. There is usually some simple 'trick' to reset these things - on one other vehicle it's locking with the fob, and unlocking the driver's door with the key. Try calling a Chevy stealership and see if by some chance they know the manual reset procedure, and are willing to share it with you.