If the cube texture of the probe is not attached to at least one reflectionTexture property of a material, it won’t be generated.
So, you can either set pbr.reflectionTexture = rp.cubeTexture or instruct manually the system that the cube texture should be generated by doing scene.customRenderTargets.push(rp.cubeTexture);:
https://www.babylonjs-playground.com/#4R1H1U#9
Regarding tinting the sky, you can use the vertex colors of the cube:
https://www.babylonjs-playground.com/#4R1H1U#10
As for your third question, you should be able to find a tint color that matches a night sky. As for adding the moon itself and some stars, I guess it’s not supported yet (but PRs are always welcome :).