The manual will tell you how or you can adjust the number of credits per coin, it was usually 5 plays for a quarter, 3 for a dime, one for a nickel,
Yes you can use non Wurly wall boxes, but the 2600 is a 200 selection juke (50 records 2 songs per 45), so you wouldn't be able to play half the songs with a 100 selection wall box.
The wall box cables will plug in to the stepper unit which passes the credit selection impulses to the credit mech. And yes you can have more than one stepper attached to the juke.
The remote volume control is just that. It was used in bars and restaurants where the sound needed to be adjusted based on how loud or quiet the room was. Do you need it? Not really, but having it is a personal choice.
And i think pop machines and jukes go together, it's an appreciation for a machine that was useful and had beauty my non collecting friends just don't see.
For the missing coin mech cable you can try
Durfee Coin Op or
Roy Dean's Jukebox Junkyard, either place can direct you if they don't have one.
I'm sure someone wil jump in on the free play question, there is an Ebay seller from Canada tghat sells a plug for earlier model jukes, I imagine it energizes the solenoid in the credit selection to keep one credit active . I'll look up his name, he may have one that will work on your box.