1. No. Jumper the "credits remaining" switch on the credit unit to permanently closed and away you go. Make sure your credit switch wires aren't shorting or you'll get the score motor continuously running, and credits continually adding, if that circuit is powered at the given time.
2. Might not be worth the effort especially as a newbie. They're a pain in the ass. None the less, since this is a later game that doesn't take dimes or nickels, they're easy to come by due to the arcade period after this which of course also used quarters. Millions of those are around. Check eBay for "quarter coin mech". The metal ones would be original but the plastic ones may even fit. (You'll need to assure both fit, no help there, sorry - use at your own risk) You'll also need to replace your credit switches as they are gone too. Looks like an annoying task but if you want to do it, understandable, it's a labor of love.
3. Dollars were used during the solid-state (computer) era. Those coins were made from 1979 to 1981. 1979 was when Gottlieb rolled off their last EM which is insanely rare and was probably never ACTUALLY released, while Bally and Williams were already light-years ahead of them and had been making solid-state games for years (and assumably Stern as well, who had bought out Chicago Coin a few years earlier), so needless to say they were definitely were NOT making any EMs anymore. So, this slot wouldn't be applicable to your machine and would likely behave as a quarter slot, and that's one you put the EM connector on it rather than the solid-state one. Between machine, a lot of them are the same and a lot of them aren't - meaning the connector in particular, not the door - they're pretty standard. Most of the Stern style doors that LOOK exactly the same (minus coin slots) should fit, just use your judgement, as no, it doesn't get more complicated.
Compare wire colors and each door's connector and if needed rewire the NEW door to match the OLD one. DO NOT rewire the cabinet, please.
Also, if you're taking the easy route as mentioned and replacing the door, keep "Chicago Coin EM coin door" in your search as well. Because when Stern bought out Chicago Coin they stole their coin door design and they'll be a direct match as well. I think Chicago Coin even used them on their EM arcade games so that's even more of a population you'll have to your advantage. They were all pretty crappy games (the Chicago Coin ones, not beating on your Stern necessarily) - so they get parted out a lot too, another thing to your advantage.