What you found is more or less the best process. The vital step you haven't seemed to mention is the use of a 2nd UV channel. By using a 2nd UV channel, you should not have to change your materials or their application at all. The 2nd UV channel is what will get 'automapped' and Render-to-textured, and it will only hold…
you can detach the parts of your mesh that are unwrapped in the 2nd channel, copy it's 2nd channel to it's 1st channel (menu -> tools -> channel info), then attach it back to the other half of the mesh.
can you use a 2nd uv channel for the normal and tile the 1st channel for the diffuse while having unique uvs in the 2nd for the normal map? or vice versa if it support multiple uv channels.
malcom the 2nd uv set is not auto-uved, There's an auto-uv tool but that's only used if an LD finds an old asset that doesn't contain 2nd uvs and they dont want to manually create UVs