You can keep a single smoothing group and the UV's, and bake the normal map in 3dsmax: decimate the high res from zbrush, import in max, triangulate the low poly, bake (without a cage), then preview the baked normal map in 3dsmax (it should be ok) or in Marmoset (setting tangent space to 3dsmax for the imported mesh). The…
In 3dsmax it looks like you have to disable gamma correction (or load the normal map with gamma 1.0), enable 1-Light not 2 in viewport settings and flip the green channel. Also make sure bump value is set to 100 in the material, not default 30. The seam might be cause by tangent space issue (baking in one program and…