I feel your pain! I have a similar problem with the hatch on my '89 740 wagon. All of the other doors lock and unlock with the central lock system - only the hatch is the problem on my car. (Sounds like you have two separate problems - the central locking system and the rear door latch/lock.) I was able to loosen up the latch on the hatch by spraying some aerosol brake cleaner up into the latch on the bottom of the door, on the lever that you lift with you hand to open the door, and in lock cylinder itself. After spraying a generous amount of this cleaner into these areas, I then sprayed a silicone lubricant in these same areas. Now, the lock cylinder will unlock with the key and the latch will close and latch properly when the door closes. Also, the key will now lock and unlock the door. The hand lever used to stick badly but now is working better. I still have problems getting the central locking system to unlock the rear door. I usually just use my key to unlock when necessary.

If you try this, be sure to protect the bumper and license plate from the cleaner - it evaporates quickly but don't let it set on these areas. If this does not work, you will probably need to take off the inside hatch panel and clean and lube the linkages. You will probably still have a problem with the central locking system working the other doors as this sounds like a separate problem.