As far as being able to open the rear door, even when the lock button is down, check the long linkage rod that runs from the inside handle to the latch. you will have to pull the door panel to get at this. I had a similar problem on my driver's door. Even with the door "locked", I could open the door from the outside by simply pushing the thumb button.
I found that the rod end had slightly bent beyond 90 degrees. The ends of the rod are bent at 90 to engage the mechanisms, having one bent a tad too far throws the works out of synch, The door will appear locked, but the lock hasn't engaged. Although, it just dawned on me that this may not apply to rear doors, as they should also be equipped with a spring joint for the rear child proof system.
But still worth a look.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.