I've made minor progress recently...
I reverse engineered the engine object spawning system I still haven't gotten it spawning objects yet though I did get it to spawn them to memory (just not place them on the map) trying to figure that part out.
Though the good news is now that I've found the area that I have after some reversing, time and patience I should be able to effectively disable the second player which I'm hoping will also remove the 2nd screen and in doing so will remove the rendering issues due to there being no overlay.
After that I would simply spawn the players manually for additional players joining the game.
With reversing the object spawns I also managed to find the weapon errors, pickups, and etc rendering function so I was able to produce custom text rendering straight to the screen which I'm sure will be useful once I want to display "Player joined", "Player killed X" or etc.
I leave you with a demo of text rendering: