24v AC common/ground
The intent of grounding the chassis isn't to help with troubleshooting.
People are assumed to be at ground potential and/or connected to ground somehow by virtue of the inability to hover. The chassis is grounded so that if a hot wire gets loose inside and makes contact with the chassis, the current will take the easier path to ground through the low resistance metal chassis instead of through the human, who is at ground or in contact with ground, touching the chassis.
If the chassis was not grounded and a hot wire got loose inside and made contact with the chassis, now the chassis would be hot. If a person, presumed to be at ground, touched the chassis the current would go to ground through the human.

Wouldn't grounding the common, which it obviously doesn't, unless in the event of a short, cause unwanted voltage to continuously energize the chassis or just act as a short?

Ground (or earth) is assumed to be an infinite charge sink. So if you short the hot line to ground, the hot line gets pulled down to ground instead of ground being pulled hot.

Is the voltage lost/consumed through resistive loads in the circuit, such as contactor coils?

You could think of this as so much current flows through the resistances that all the voltage is dropped across those resistances so by the current reaches the chassis which the person is touching, there is no voltage difference between ground and the chassis with which to shock a human at ground potential or connected to ground.