Hi there,
That model car has a contact on the starter solenoid to bypass the current dropping resistor in the supply line to the transistorized ignition module when starting, it could be that the contact isn't making when the starter is engaged, which would make for a weak starting spark.
The distributor actually has FOUR sets of what Mercedes calls "release" points under the ignition points plate. They cause the injectors to fire in pairs of two. If one set of points doesn't make contact, the engine will miss on two cylinders because two injectors aren't working.
I would jumper out the ballast resistor while cranking and see if the spark is then hot enough to fire off the engine. Don't leave it jumpered as the ignition module could be damaged if it is left jumpered for any length of time while running.
I wouldn't be afraid of the earlier injection, it works well when in good shape.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C