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This should already be there under blending, it's the third option. Just click all RGBA buttons

This has now been fixed in 1.27

The same way. Create a texture asset node

Not sure why that is I'm afraid. No updates on when/if it'll get fixed. I wouldn't wait for it! You can manually check the shader code to see if you can find out why it doesn't work.

You can't just add them, you need to do some triplanar blending based on normals as well. Google for triplanar and shader forge, and you should find some tutorials!