How can I make it so when I press Middle Mouse Button, it will slow down the NPCs? I already know how to change their speed in the config files btw. (Also, this my first time modding)

I'm don't know much about B3D, but would it be possible to make it edit the config file then reload said file?

You would need to modify the Source Code of the game using Blitz3D, then recompile the code as an .exe, to put it simply. The exact details of how to do this are unknown to me, so don't ask.