Yes you can replace the diaphragm with the pump in place. I did it on my 78 240D. If I remember correctly, you dont even have to take off the fan an belts off if you have the right tools. But it will much be easier with them out of the way. The plate is held on by flat head screws. I found that a flat head driver dip in a socket and ratchet works best.

If you're not getting oil in the vent line than I wouldn't think a new diaphragm will fix the problem. I would suspect the check valve on the pump for the line to the brake booster, or the internals of the pump. Both of which can be replaced. If it were me. I'd pull the pump and to a total rebuild. Then you won't have to mess with it again.

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