At what time during the assembly process was a car undercoated? Were the wheelwells undercoated at the same time as the body and was a vehicle undercoated before or after the drivetrain was married to the body?

I'd say most of the bolt-on items were already on the underside before the undercoating was sprayed. The brake and fuel lines were installed along with the gas tank and muffler heat shields.(E-body) The rear swaybar on my Challenger was already installed. It was probably done right before "body drop". The seats were installed before undercoating too.