All wiring I've ever seen in pinball machines has been silver in color.

I have a switch wiring in the manner you speak of (silver -> copper) for a few months and have no had any issues, I soldered them together and finished with shrink tubing. I would not/do not leave the machine plugged in all of the time anyways (and highly recommend against you doing so either ) so it is not as important as house wiring.

Not true, games of this era have both a "120V hold" relay and a "30V hold" relay that need to be activated (are these both 30V driven? I think the 120V is only through the switches but at the same time that wouldn't make sense), and 120V through the start sequence. Also the same as games a few years newer (or if it's add-a-ball then especially), putting in a coin in the left slot will immediately start a game just like the start button would except without subtracting a credit. So with that paired with the age (120V vs. 30V), there is always some voltage at the credit switches.

I know you said "outside of the door", but I assume you meant that the start switch is always hot (which it is) and the credit switches are not. However, they would have to be too as they "light up" the game (Gottliebs of this era are FULLY dark, then later when the 120v hold relay was removed and only the 30v hold relay remaining, they are only half-dark when first switched on just like they do if you manually activate the 120v hold relay) and kick it into the start sequence during the first start after the game is turned off.

Just, "better safe than sorry". Advice is risky when dealing with that kind of voltage. Not 100% sure if I'm correct but I mean for a switch to work like that, there must always be voltage going to it (constantly, and not even just when it is "on", in the startup state it is as well), and I'm fairly certain it's 120V on this era of EM.