looks like a good plan....
first, to answer your questions:
the ipac 4 has 56 inputs, each joystick = 4 inputs, so 4x4=16 + 34 = 50, so you still have a few inputs left over.
you may want to do some reading on the magstik. it seems as if alot of people buy them because they are switchable, and end up replacing them because they dont like the feel of them. also, the magsticks will look physically different than the happ sticks.
if i were you, i'd either get U360s for player 1 and 2, or get all happ 8way sticks, and install a dedicated 4way (ultimarc J-stik makes an excellent 4way)
as for admin buttons...it is my belief that Less is more. i'm not sure what exactly you are planning on with the UP/Down/list buttons, but those can all be accomplished with your existing controls. for instance, instead of up and down buttons, why not use the joystick up or down? map one of your regular buttons to change lists. pause and exit are must have buttons, but Start is not. most front ends will use the player 1 start button to launch the game, so you wont need a dedicated start button.
also, make a cardboard mockup of yoru panel to make sure the layout works for your.