Actually, for a four-player panel, it's a lot of compromising to use a KeyWiz alone (but possible).
That's what I was thinking too, and was just going to suggest the I-Pac4, but then I priced it out at GGG and at Ultimarc- with the "One Click Shazaam! Adaptors" and one KeyWiz Eco (solder), he can get up to 56 inputs for less money than a 56 input I-Pac4. More work, but cheaper. (Always a trade off...)
No you can't. (Theoretically, it's possible, but . . . more below). What you can do is get a KeyWiz and a GP-Wiz for less than an I-PAC/4 and 64 inputs.
With a KeyWiz alone, the best you could realistically do is 32 action inputs and about 10 shifted inputs (for start coin and maybe Pause and Escape), although I would recommend not using Pause and Escape, so really 40 inputs.
The reason your idea (56 inputs) won't work is that pressing a shifted input shifts all the other inputs, so they can't be used for action keys.
For example - let's say I have P1B1 shifted to register Coin 1 and PB2 Shifted to register Start 2. I am playing a two-player game and P2 presses the Start button. P1B1 will now add coins rather than firing until the P2 Start button is released. And this is with the shifted buttons as admin functions. You have more problems if they are mapped to action keys.