Removing the aftermarket alarm was a good idea, they tend to be troublesome on these cars. Be sure the wiring is restored to it's original state where the alarm connections were.
It sounds like corrosion is going to be the final culprit here. I think the module is getting intermittant signals to lock or unlock via a bit of crud conducting enough electricity to make a brief connection.
Since you've found corrosion already start your search in that area, connections first, then pop apart the GM and have a look inside. There are write-ups here regarding the repair of the GM.
Sorry you spent for non diagnosis at the dealer. BTW it takes only seconds to swap a GM or relay, not hours. They are supposed to be EXPERT hence the high fee. WTF did they do for over 6 hours I wonder?
My bet you will find something in less time. Good luck.