Go for it, however you will need to import the images into Blender so that you can choose it in Texture Remix. Think of it as your own images you bring into Blender for which you want to paint onto the mesh

No, all remixes will transpose onto the selected base UV Map you choose for the 3D mesh. This means you can unwrap the way you like, then leverage the abilities of texture remix to paint multiple angles and the resulting image will be on the layout of the base UV Map.

No, the purpose of Texture Remix is to 'paint' an image onto a mesh that has UV maps, this can be an existing image you might have