There seems to be code that would initialize the mouse device through DirectInput, but it is unused. I put a breakpoint there and the game never passes through it.
Ok

So I tried to find the "if condition" to initialize the mouse, and the condition was a boolean value in the CKInput class, and this value is actually serialized in the GAME.KWN. That means you just have to modify a single byte from 00 to 01 in the GAME.KWN file to enable mouse support.
But so far only the buttons seem to work, you can't map mouse movement, and for some reason left and right mouse buttons are swapped: If in the input settings you set a command to the left mouse button, it will say "Right mouse button" and vice versa.
Yeah, just like Left-Right and Up-Down for the right stick are inverted with fixed DirectInput controllers. Thankfully, a text edit can bring some sense back.
Anyway, very cool discovery Adrien! You amaze us everytime. So, this byte change should be permanent? Another great thing to add to the patcher.
It's in GAME.KWN at byte 0x25A. But it's always possible patch that directly in the GameModule or via DLL.
Mouse movement does work, but only in fullscreen. And, not sure if it's because I only tested with my laptop's touchpad, but the movement is very slow, when I mapped the mouse movement to the camera.
Touch Pads need much higher sensitivity to compare with mice
When playing FPSes my tp is on 20 sensitivity, Minecraft is on Max
I'm starting to believe, Adrien, that yes, a custom dll injector could be quite useful. Hey, what if all the gamemodule edits were on your dll, so that the gamemodule remains vanilla, aka non virus positive?
Yes, that's what I was thinking: have a DLL injector that would patch the game in memory.
So installation you just require copying a DLL to a folder, no exe patching needing.
Plus I could make the injector load a settings file, so you could enable/disable modifications to the game individually.
Something I already did for Warrior Kings

THe game would lo longer be AV flagged
Altough maybe the language mods should be actual file replacements
( Unless you really can pull out that aswell )
And yeah, a settings.ini or .txt could help us set options with true/false and other things