JamesGV:
As a way of "resolving" this "log off and Log on" issue which is believed to have occurred at "some time prior to 1825 UTC" and "at 1825 UTC", you are saying with your time on T7's it is possible to "force" a "log off" without de-powering the system...and without leaving the flight deck.
Assuming you did this. The system has power but is "off line", how would you "log on" again ? Would this be something that would be "automatic" ? Or a "manual" function ?
Sorry, I neglected to answer your question. Once logged off, the flight crew would need to log on again through one of the flight deck CDUs. There are two types of log on available - auto, where the system will log on to the best available satellite; or manual (constrained), where the system can be constrained to log on to a particular satellite. I could be wrong, but I think that once the system is manually logged off, it will not log on again until one of those options is selected.
Note: I am only trying to explain how the system works on the flight deck. I am not inferring what may or may not have happened in the case of MH370.