Ok, but the ground strap terminal is contacting the firewall itself, and any fastener will suffice as long as the firewall metal is cleaned to allow for good contact between the terminal and the firewall.

The wiper motor circuit has nothing to do with the grounding we're trying to achieve here.

 

Not very well if it is secured to the stud through the rubber bushing.

 

Right, and if youve tightened it to the point where it creates good contact, you may get vibration when running the wiper.