Jim,
With regard to the "Paris scenario", I don't think AC would be able to differentiate between a sign-off via log-in and one done locally by your partner. On the other hand, logmein does create a log which would record when you were signed in (and from what IP address). So...in the above scenario, with a little research, your partner's legal beagles could match the date/time of the sign-off with a time that you were signed into your computer with logmein, and voila, he is off the hook.
Certainly that is not the ideal mode of documentation, and it is for that reason that I might not favor the "shared password" approach, but still it is a reasonable fall back position.
