* I got out a KE-Jetronic Bosch manual and looked for that spring on the central plunger. You're absolutely right! There it is. The Bosch manual says that "In some versions a pressure spring is used to assist this hydraulic force (line and control fuel pressures). It (the spring) prevents the control plunger from being drawn up due to vacuum effects when the system cools down." So the good news is that the spring is an engine-off assist, and that you could probably adjust out the pressure-effect difference, with the engine running, caused by the spring (if you couldn't find another spring).
* I would try to find a core fuel distributor off the same model MB. Tell the dismantler/junk yard/independent that you don't necessarily need a FD that works because you just need that spring. Then again, if your initial problem was fuel distributor related, a good used FD should take care of both problems. Don't forget the o-ring and the fact that you'll need to reset duty cycle (idle mix).