Wow is there anything you don't have a prototype of yet. Your in depth thought and dedication to the project is astounding.
If what you are referring to is having multiple players playing off of one computer, then the reality is that your options are quite limited, as of now, there is only one piece of software which has ever accomplished the multiple mice on one computer idea, and it does so by directly hooking the mouse drivers and creating multiple cursors via software while simultaneously overriding the default operating system cursor. The implementation of which I assume is not worth the minor feature it would result in.
Touchscreens are however of course an entirely different thing what with technologies like multi touch.
TL;DR Yes, it is quite difficult