You can see the individual GCode lines if you connect a PC via USB and send M111 S1 P3 to enable GCode debugging.
Single stepping is a facility that I already have under consideration, but is low priority at present - I can see that it would be useful, but you are the first person to request it.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
