NEX7 firmware still buggy...

I find the MF Assist feature a better workaround than DMF.  After the camera has AF on the subject, I switch to MF (by pressing the AF/MF button), then finalize the critical focus in MF Assist by turning the focus ring.  This alleviates the need to keep the shutter button half-depressed in DMF.  An additional bonus in MF Assist is the ability to control how long you're magnified on the subject, depressing the shutter button halfway "un-magnifies" the scene... allowing you to go back to recomposing the picture when you're ready, thus independent of the delay timer.

Give it a shot.  You may have to set the AF/MF lock button to MF... I can check the settings on my camera, if interested.