For question # 1, the way to test the efficiency of the catalytic converter, we do a pre-cat ( before converter ) & then a post-cat ( after the converter ) test.
That's why OBD II vehicles have two O2 sensors.
# 2 , to test for an exhaust leak, plug the tailpipe & see if you can hear any hissing sounds in front of the O2 sensor. If this is done on a cold engine ( first thing in the morning ), you would possibly see some condensation cloud.
# 3, I am not very familiar with all the M-B engine controls specifically, therefore I tend to talk in " generic " terms.
# 4, other than running your diagnostic check, which you have scheduled for the 24 th of this month, I would postpone chasing any more demons until then.