I think object space normalmaps don't use the normals of the lowpoly at all just override them but tanget space works together with the lowpolys normals.
Hi! No problem using separate files for highpoly and lowpoly meshes to update them separately. When using "Quick Loader" in a Bake Project, you can add multiple files.
Does the lowpoly shading look good with the normal map applied? Hard edges are certainly useful to control the shading of the lowpoly, especially at steep angles - just remember to split the UVs along hard edges to avoid artifacts! Maybe check out Making sense of hard edges, uvs, normal maps and vertex counts?