i have no answer to your code(why diffuse is twice, i belive its not diffuse, but other texture like normalmap, since it also have scale as i remember). But day/night cycle is so easy to make. I dont see problem.
Level easy solution: You can just change AmbientLight/DirectionalLight params in-game.(low for night, high for day)
This is the standard solution for day/night cycle along with skybox changes(if there is skybox)
Just lower intensity of it and its all, why not?
Level hard solution: but if you want disable directional light, then just do it in shader. Just disable it.(using Material parameter you can setup in-game like “useDirectionalLight” for material)
Editing shader is not so hard when you learn basics.
Anyway i suggest you to use modern version:
that support PBR material.