Hmmm, Looking to build the ultimate cabinet, good luck with that one, People have been looking to build that for years, Someone came close once with their ark of the covenant inspired build, Just for the love of humanity do not build a chase designed control panel
I would personally recommend a pedestal unit with the largest TV you can fit to the wall.
1 ) Look at real 4 player arcade machines, Simpsons, Xmen, Gauntlet, just scale up a little bit to allow room for the trackball
2 ) Yes there are lots of issues with mapping and swapping controllers frequently, but there are also a number of tools that can assist.
3 ) As per pedestal comment
4 ) For HD Gaming I recommend something along the lines of I5 9600K GTX 1650/1660 16GB Ram, 512GB M2 Drive, Largest mechanical drive possible, 500W PSU
5 ) Hyperspin is good if you are willing to put in the time, don't just buy a drive full of roms and expect it all to work, Personally I used Bigbox/launchbox for my last setup having been using hyperspin previously
6 ) Can't comment on this, I've always been happy enough with wii remotes as guns

7 ) Happ/Ultimarc Trackball
8 ) Ultimarc spinners, I believe that they also do the little steering wheel attachments, someone did do this previously
In short build whatever makes you happy, but I would recommend something like this
https://www.pinterest.co.uk/pin/377809856212021222/ or
https://www.instructables.com/4-Player-Pedestal-Arcade-Cabinet-for-MAME/and not something like this
https://www.pinterest.co.uk/pin/529665606157952822/