You have not stated which side of the car is problematic; all
coupe` C123's have some hand-work at that chassis joining
but remain quite similar. If it is the window lift itself that you
ask about then yes, they wear over time and require replacement.
Now the 123 coupe`when operating properly will reveal a
slight overlap of the rear window against the large passenger
windows. This is as was envisioned by its architects orginally.
An attempt to sealout the elements is gained via rubber welt and
other fabrications. Over time as that coupe` ages it will show
signs of increased warpage and weftages, but should not alarm
unless wind and rain enter the passenger compartment. Your
last notation in re access is surmised correctly but removal of
the rear bench bottom must be done; a skilled practioner with
many excursions to that area wont find it necessary to remove the rear bench backing, but a novice will.