Facet edit, even in V11, lets you select the outer and internal end faces of a shelled cube and just move them the distance you want, without the slicing and adding, and whatever else you did, which is pretty much unnecessary makework. I wouldn't recommend facet edit if you were using the part tree, but if you were using the part tree, you wouldn't have to slice/add/etc to make that change anyway... In V11, you can create that hollow box by NormalExtruding a rectangle (compound profile checked) with a line or polyline or curve for a path and shell it, or even fillet the edges then shell it. Making the path longer makes the extrude longer, filleting/shelling are persistent features that remain even if you alter the size of the rectangular profile or length and angle of the path.
When you perform booleans, try to have overlapping parts instead of having them meet exactly. Having faces coincident in space is called a "degenerate" condition in CAD. It causes logical errors within the 3D kernel, which you've seen the result of. They've developed ACIS a lot and it's not nearly as common as it used to be, even if you do create degenerate conditions.