hello 
As for the math on mobile…
It depends on your target mobile phones/tablets, you can use some math, if its very limited (and if you do, but any math you can avoid, the better of course.
And since you are catering mobile, anything you can do to optimize or avoid math is king.
Customized UV’s are great for this kind of thing… but how to set them up?
I’d check: Customized UVs in Unreal Engine Materials | Unreal Engine 5.2 Documentation to do so
And the “three texture samples in use” isn’t only for texture samples you place.
Depending if the shader is set to tangent space normal maps or world space normal maps can save you 1 texture sample, as can unlit… but I dont know enough about this to give more in-depth advice.
It might be “smart” to look at the free infinity blade content on the marketplace to see how they solved it.
As for using a separate texture in each channel, that is a really good way to go and I try to use it wherever possible.
In some cases I will add the roughness or other grayscale texture (metal mask/AO/emissive mask) in all but the alpha channel or store them in a separate texture using each of the RGB channels for those aforementioned grayscale textures.