Great questions and advice. I like the split cab idea. Might not go with that, but will definitely consider.
For question 1, the main reason we wanted this in the first place was for the 4-player games. So yeah, 4 player is the focus and multiplayer is a big thing in my family.
I hear you on the frankenpanel cabs. Our current plan is just the four joysticks, one trackball, and two guns. Spinners seem way too expensive to be worth it. And I wonder if an analog joystick can't do a passable spinner impression in some games?
I want it to be able to play as many games as possible with the fewest controls. We are focusing on arcade games, so that does relieve a lot of requirements. I don't mind if we can't play the games at a professional level, as long as the bulk of them work well. I'll settle for good over perfect if it gets me flexibility. From what I have read, the U360 can do most games at least "good" if not "great". Mediocre, though, would be unacceptable.
But yeah, even though they aren't the main focus, I don't want fighting games, for instance, to play poorly. Can you still pull off special moves with a U360 set to 8-way with no restrictor plate? (I haven't decided if I will do them with a restrictor plate or not- though if I do, it will be round, of course.)
I am also not sure on how far back to tilt the screen. 4-player cabs tend to tilt back pretty far (we are thinking 32inch LCD at this point) but might that interfere with the gun games?
I saw one well-received project blog that used two U360s and 2 standard 8-ways. But one of my favorite games was Super Off Road and I'm pretty sure I'll need analog joysticks to simulate those steering wheels for three players.
That same project also used 8 buttons for the first two players and 4 for the other two. There aren't any arcade games that require more than 6 for 1 or 2 players, are there? Most designs seem to stick with 6. I think this guy was also putting console games in his, which we won't do.