Ya I was probably going to rebuild the top of the control panel to add in a layout that I like unless I decide to keep how it is not and just add more too it. But yes 6 buttons per player then any other additional buttons for menu, start, play coin, back etc.
You may not need all those buttons.
Start by installing your front end and emulators and see which ones you really need.
Not sure what "play" is for, but change that to pause and it makes more sense.
Many people find that P1/P2 Coin, P1/P2 Start, Esc (exit/back), and Pause are good admin buttons. YMMV.
Depending on your circumstances, you
might consider a Tab (menu) button.

Less admin buttons is better. :p
You can definitely do without menu, start, coin and back buttons.
Spoken like a true-believer adminophobe. :p

Usually player 1 button 1 will start the game.
If you mean launch the game from many front end programs, I agree.
If you mean starting gameplay after the game loads/initializes and you add a credit, then on most games it won't, unless you remap the P1 Start button default.
BTW, hope you don't plan on playing S.T.U.N. Runner the 2nd way since the start button is also the trigger for Shockwave.
Menu is an admin function, ad shouldn't be accessed by a normal player. Use a keyboard for that.
Depends on whether there will be nosy kids or drunk/clueless friends using the system, which depends on OP's situation and whether he likes to mess with the settings or not.
Back and coin can be tied to an encoders shift function. This allows you to press a button to enable a second mapping and press another button at the same time.
Shifted functions can cause trouble if you're not careful with the setup.
If you and a friend are playing TMNT and he does the shifted function for coin at the same time you are pushing your joystick right, the IPac outputs "Tab" and the menu pops up.
Dedicated inputs for P1/P2 Coin and P1/P2 Start combined with disabling or limiting the number of shifted functions make it far less likely that you'll accidently trigger an unwanted function during gameplay.
Scott