This whole 1st gear/2nd gear start thing has me wondering if it's related to the optional ASR (automatic slip control) that was available on 300E's for a certain time period. (on my '91 at least)
My car did not come with ASR, but it seems counterintuitive to put 1st gear start on a car without this feature.
Just a thought.
'91 300E, 214K miles