By default, Qimage reports my printer resolution to be 360dpi, even when I set the resolution in the printer driver to its "highest quality" default. Only when set the quality option "finest detail" on does qimage recognize the resolution to be 720 dpi. Is that correct?
Yes, this is correct
Also, when I set the printer to "borderless printing", Qimage reports the printer resolution to be 738dpi. This is with the Expansion option in the printer driver set to its default "Auto Expand" mode. If I set the Expansion option to "Retain Image Size" in the print driver, Qimage reports the resolution as 720dpi. It also reports an 8.5x11 sheet of paper to be 8.894x11.394. I assume that's because Qimage is setting a "fake" white border around the 8.5x11 image. What is the right/best way to print borderless? I assume I should use "Retain Image Size"?
Everything you describe is normal but Qimage is not setting a fake border. I don't have a 3800 to make a print, but The driver is doing the changes. Qimage is just reporting the changes.
I think, and this is a guess, that Normal expamsion focuses on the edges being stretched to fill the page.
I think retain size is where the driver is fooling with the printable area making FIT TO PAGE think it is making a larger print than the printable area of the paper, and you end up with a fully printed sheet.
Try making a print using both methods, and pay particular attention to the print area that is close to the edges of the sheet
You will likely see a little distortion on the first method.
Fred