By way of an update, it is likely that the culprit is a check valve in the secondary pump assembly. The working assumption is that that check valve came apart and the spring from the check valve got mangled in the pump gears, which in turn took out the governor gear.

An MB master tech gave me some guidance over email...do you think my mechanic will give me a "research discount?"

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife