My W210 wagon has the same problem....2nd row passenger seat latch stuck and seat won't fold down.
I don't think that the mechanism has ever been lubricated...previous owner leased the vehicle.
Is the repair process the same as the W124?
2012 ML350 Bluetec 4-matic
1999 E320 4-matic Estate
"A ship in harbour is safe, but thats not what ships are built for". - John A. Shedd