1) Why bother wasting the space for two "insert coin" buttons? Player 1 and 2 I understand of course.
already been said, some games require independent coin buttons
2) Some people include Pause and Exit buttons for MAME, some don't. On others, I don't even see an Enter(Select) button! How do those people choose their MAME games to play without Enter?
as for choosing a game, most front ends allow you to customize which button you want to use to start a game, i use the player 1 start button. as for pause and exit, i like them. on my first cabinet, i have both pause and exit, on my second cabinet, i have 1 button that is pause if you press it quickly, and exit if you hold it down for 3 seconds.
3) What is the customary way of scrolling through the game choices on a regular control panel layout? Does the primary joystick double as the arrow keys?
i use player 1 joystick to scroll through games.
4) Very few control panels seem to include TAB(options), VOL up and VOL down for the speakers (let alone not including Enter or ESC). What do people do in these instances? Plug a keyboard in every time you want to change a game?
i have a volume knob on top of my first cabinet, my second cabinet uses a TV, so i use a remote to adjust the volume. as for TAB and other admin type functions, i use button combos for most things, and a usb keyboard for the more in depth stuff. for instance, i didn't want a tab button on my panel so all the idiots/kids who play my cabinet cannot screw up my config. so i configure mame so that i have to press p1 start+p1b4+p1b5+p1b6 to bring up the TAB menu...that way i can access it easily when i need to make a change, but it is difficult for someone to accidentally do.