Get mask from a dot product including a PixelNormalWS on a normal map ?

Hello, I was wondering if anyone could help me on an issue I am having. I am following a video tutorial on making a planet (video here : https://youtu.be/o_3Kv16GIoA ), which I have no issue with, however I would like to use a normal map to show the relief of the planet but I cannot find a way to get the result of the dot product + PixelNormalWS I am using to generate the shadow on the planet and use it to mask the normal map so it doesnt show on the shadow side of the planet. See Pictures below:

Here is the result I get from the Dot Product

Here is the unmasked normal map and how it affects the planet (please disregard the broken seams etc, I’m just trying to make the material work as intended for now)

Planet without normal

planet with unmasked normal

Result I would like to achieve (collage is PS)

I Thought I would be able to use the result of the DorProduct as a mask for a Lerp, since the lerp interprets the result correctly, as seen below

But as soon as I connect the Lerp to the normal input of the material I get an error and the material doesnt render at all

So I would like to know if there would be a workaround or any other way to achieve the desired result ?
ps : I cannot bake the shadow directly into the normal map as several maps are animated and need to move independently from the shadow.

Thanks a lot !