These are wrong
Your normals are fucked because that's how normals work... You have a 90 degree corner there, and smooth normals aren't going to handle that well. If you want an edge to be sharp, you need to mark it sharp. That essentially creates two separate normals.
That's what this:>>536620
These are also wrong:>>536641>>536695
Edge-split is intended for exactly this scenario and it does not add polygons.
Is good advice. The edge-split modifier will actually "split" the edge when applied, but there's no need to actually apply it.