Stan, good question and those are legitimate concerns, heres my engineers perspective.
To lay persons, calling what most people call the GROUND CLAMP makes sense to them just as they call a tractors frame GROUND. You may confuse an untrained person if you go over their head trying to define terms that engineers and electricians and electrical techs commonly use. I call my welders ground clamp a ground clamp and did BOTH before and after my engineering career and call the other the stinger (electrode holder) BUT ALL ARE FREE TO CALL THEM ANYTHING THEY PLEASE LOL
Ifffffffff youre talking about a cheap basic Lincoln Tombstone or other AC Buzz Box Welders, they are basically a simple isolation transformer and their output voltage is across their leads. The safety Equipment Ground is bonded to the welders metallic case to provide an independant dedicated low resistance path to carry FAULT CURRENT ONLY NOTTTTTTTTT Neutral current.
As far as voltage potential from the ground clamp or stinger to mother earth or safety equipment grounding conductors or gas or water pipes or building steel or a concrete floor that depends on the welders design and the branch circuit wiring and length of wire (capacitance or mutual inductance) and I just cant say what it might be BUTTTTTTTTTTT I DONT LIKE THE IDEA OF TRYING TO USE OTHER THEN THE WELDING CABLE AS ANY RETURN PATH even though in theory (depends on welder) there should be low potential and low current.
I cant speak for my friend the good Buick man, but the reason I tell people to NOT use the GroundING conductor for the Neutral (A GrounDED Conductor) and vice versa or to mix n match or swap or substitute one for the other IS TO EDUCATE THEM AND SAVE LIVES but I also say do as they well please, Im here to try and help maybe save a life, NOT argue or tell them what to do
John T Long retired EE