The Layer Blend node itself should not be a problem. There was a limitation for 3 landscape layers on mobile, but as far as I know, this does no longer exist. So having material functions fed into the layer blend should be ok, then using a Break Material Attributes and feed the required outputs into the material node.
Note: In the above case, I used a Landscape Visibility Mask in addition to cut holes into the landscape (useful for caves etc…)
Not sure about the Height you are returing from your material functions. This makes me believe, that you maybe are going to use some tesselation later on. Tesselation input is not supported on mobile platforms, so this might be causing the problems.