I did look at the 720 input and output file and, as expected, there is a definite difference in line thickness between the 720 ppi and 360 ppi in column D. This is true whether the print driver is printing 8-bit, 16-bit, 1440 or 2880 or 2880 with fine detail on or off. One difference I did note in printing with finest detail off is the 720 ppi rectangle of Column A is a lighter gray to the naked eye than the other ppi rectangles when finest detail is not selected and I cannot make out any hints of vertical lines when viewing under a loupe (I just see darker spots splotched on top of a lighter, seemingly solid background). With finest detail on, the 240, 360 and 720 rectangles are all darker to the naked eye, and hints of vertical lines may be seen in the 720 rectangle, though not clearly defined.
On the image size front, yes, I could not get the image to the correct DPI within LR when working with US measurements. In PS, I merely changed the canvas size to 6x4 inches which did nothing to the image's ppi. In LR, I then set the cell width to 6 inches and all was fine.