1) as said, buttons are personal taste.
My preference is for just the needed game buttons, 1 & 2 player start as well as pause and exit.
I prefer to not have a bunch of admin buttons taking away from the arcade feel and look.
2) the power goes through all that but it's not like it's a power supply powering all that.
Easiest way is to have a smart power strip where your pc plugs into the control outlet and your marquee light, monitor and all the rest go to the other outlets, when the pc gets turned on, the other outlets go live and everything else turns on.
3) No one can tell you if coin mecha ate worth it or not, it's for you to decide.
For me, it wasn't worth it to actually use coins so I mounted switches behind the coin rejects so that when pressed, it gives credits. It's a very common way of doing it.
Whether you use that method or actual coin mechs, it's similar for tieing into the system. With coin mechs the coin just hits a switch. Just tie the switch to your encoder