After the freeway drive dropped to 12.08? Something is going on as you are all too aware. The glow relay may be hanging up or coming on and dropping out intermittently. It is easy to disconnect after starting to see if things return to normal. You may yet be forced to wire a cigarette lighter socket to your meter to monitor the battery voltage and how that alternator is producing with the meter on your seat. If the drive home was with no headlights then the glow relay does become a prime suspect. If not the relay then the battery was not seeing the 13.5 + alternator output. Nothing in the car has a draw strong enough to overcome the alternator except that and the starter. The starter is eliminated of course automatically. I would monitor while driving now. Some of your voltage postings might indicate the battery is not really fully charged. Or perhaps almost overcharged with a bad cell. I think 12.56 is considered a full battery but not sure. Make sure to carry a set of booster cables in the trunk with a spare battery if possible till you locate this. I have a feeling you might need them. Make sure to have mercedes free roadside assistance phone number with you if you do not have a spare battery. Sometimes it's on the back of the glovebox door. Not an earth shattering problem but just a pain in the posterior.