First of all: no, you are going nowhere with Steam version (unless somebody realizes the magic of this or this tool)
Second: retail is really fine. You don't need cracks, "lan fixes" or anything like that.
even though for some reason my DVD drive hangs the whole computer every time I insert the disk. So kudos to RELOADED for his universal safedisc loader
Long story short, you'll need to:
- have at least a CPU core per player
- make as many in-game profiles and copies of the executable as players
- remember:
r_fullscreen 0,r_mode(and a borderless fullscreen tool perhaps) are your friends
The first instance of the game you start will be the one of the latest player.
When you start another, the previous will fallback to offline mode. In the new one make sure to select another profile.
And so on until you have as many game windows as players
The last you'll have opened (which is going to be still online) should host the LAN lobby
Now, the only thing left is passing controls to the game.
Actual solutions are tools like Xpadder (to make controller look like keyboard) and keyclone (to pass input to more windows at once)
But this sucks. So in Split Play developer I trust
Interestingly, net_lanauthorize is already 0 by default.