I also own an O2 + ODAC combo and these are some of my observations.
I can't verify anything. I am not really sure if the audio that I am getting from the line-in is actually from the ODAC or not. However, there is in fact audio that is coming from the line-in once the O2 amp is turned off. Whatever indication this is I do not know. Even when the power is on the line-in still acts as an output for what I believe to be the ODAC. The volume knob does not affect the actual output of the line-in. I think that while the combo is still connected to the computer via usb, the line-in will continue to act as an output for the ODAC.
I suppose this would be useful if you had another amp and wanted to use the ODAC as the D/A converter or you simply wanted to test different D/A converters. As far as speakers go, you will need an amp for them and I do not think directly connecting them to the ODAC which has no power output is ideal. Of course, this depends on what type of speakers you run too.
There seems to be a lot of flexibility with the combo as far as I know and you can personally try out what you think works the best for you since you already have the unit with you. Good luck.
My only concern is would any of these connections overload the circuit/headphone/speaker and cause damages?