[Import & mess with OBJ]

Objects imported into the Paint room are considered Paint Objects, while objects imported into the Sculpt room go into the VoxTree (even if they're surface.) To get your mesh into the Sculpt room, open that room and select File -> Import Object, then on the import menu select Import w/o Voxelization if you want it to remain as polygons.

[[Solved] How to force an island to be an square shape]

I don't know about 3D Coat but in Blender you can download the free TexTools addon and use the Rectify function to do this.

http://renderhjs.net/textools/blender/

[[Solved] Exported Normal Map Issue]

In blender make sure the image texture node going into the normal map is set to non-color data.

[Cannot get any results from baking]

Any time I attempt to bake from the retopo room the result is empty normal map and diffuse layers. To troubleshoot I created the simplest bake scene I could, a voxel or surface cube and an unwrapped plane, and even that fails in the way I described. Updated to version 4.8.25 (educational) and still have this problem. The same low poly and high polys exported to Handplane bake just fine. Any suggestions? I've watched several videos and have tried to follow the same steps and get nothing.