Q1: You need to explicitly define the BC type for STL files. Default walls (NSW) only apply to the MFiX box planes (say x=xmax etc), but in my opinion it is bad practice to not explicitly define all BCs.
Q2: You should not use partial or full selection here because the stl triangle edges are not aligned with the selection plane. You should use the slice selection so the triangles are cut at the specified plane locations.