I had a similar problem over the weekend with one of the sedans. I too was prepared to pull the interior door panel until I saw that the seatbelt was jammed in the door lock area.
I decided to pull from the outside while someone gently pushed from the inside and it opened. The seatbelt was crunched but not cut.
If this is not the problem then be sure the lock surround (rear of door) is off before trying to remove the interior door panel. If I recall correctly I ran into something like your problem on a past 123 model and I removed the rear seat cushion and seatback to help gain access. The seat bottom is easy and the seat back is held in by three 10mm bolts at the bottom.
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)