The reasoning for ‘Backing a Normal Map’ is so you don’t have huge amounts of geometry, this technique was done for video games, ‘backing’ is a geometry specific thing, it can not ‘read’ textures - it only reads ‘Normals’. As far as seams go, that is up to the user, or which method one uses to ‘un wrap’ the object(s).
I don’t know what the difference in geometry is between your high poly and low poly object is, I did look at your pic (knife) but its unclear which version it is, looks nice, so maybe high poly with textures.You can ‘Bake’ textures (minus the normal and bump maps, they have separate options) and apply that to your low poly object, you can also use a normal map generator that uses a pic to create a map(s), I use one from BYOB (LuxCoreRender developer) Free Normalmap Generator
You just have to pay attention to your settings (I think it makes them in reverse, but there is a check box to reverse it). Hope this helps, if not, post back and I will try to answer any questions I can.