Added 6 minutes later
It seems that the plan needs to be reworked. Just sitting and accumulating troops until the opportunity to break through the first Necropolis garrison appears, repelling waves of brown and green, and then moving all available troops (absolutely all) and surrendering native territories, relocating to Necropolis castles, and not touching Lucifer at all. The only question is: will there be enough money to buy everything if I don't establish control over the mines in the Inferno zones?
Added 5 minutes later
I looked at the save files from a 130% completion playthrough, and brown and green only arrive in the middle of the sixth month, that is, two months later. This is how the chosen difficulty level affects the speed at which the AI appears.