I believe they are one in the same. You only need one button.
For question 2, if your using the default mame config, player 1 coin insert is the number 5 on your keyboard.
Keep in mind, if you plan on doing a 2-4 player cab, there are several games that require you to insert the coin for that specific player. ie. gauntlet, simpsons, tmnt etc etc. If you are concerned about this, then you'll need an insert coin button for each player. If not, one will suffice. If i'm not mistaken (which is completely possible that i am) the default insert coin keyboard buttons for mame is:
Player1 = 5
Player2 = 6
Player3 = 7
Player4 = 8