1) the harmony database may actually contain discrete codes that even your OEM remote won't have. They're supported by the TV but were simply not added to the remote. Happens all the time, especially with HDMI and DVI inputs that were usually added to the TV lineup later and they didn't feel like changing the remote buttons. Unless someone has that model TV and can check it for you there's no way to know for sure if the commands work (they may exist in the db but that doesn't mean they actually work).
2) I believe Logitech support can setup a script for you to do this. Maybe someone who's had this done can comment.
3) You can do this yourself if 1 and 2 fail. You set up the main TV device as you have it now but you give it dummy input commands for DVI-DTV and DVI-PC. Then you create a "clone" - another copy of your TV device that uses the toggle or cycle through inputs option for input selection. Turn off power control by telling it the cloned TV device has no power buttons - the main TV device will control the power. By setting it up this way the cloned device will cycle between DTV and PC inputs and will not resend the toggle command unnecessarily - e.g. it knows you're on DTV and will not send another command. On any activity where you need one of these inputs, choose the bogus input on the main TV device and add this cloned device and select the correct input for it.
