If you're having to put it in Neutral...It's either the NSS playing with you or a
Transmission linkage Bushing (s)...Maybe Both.

I'm gonna obfuscate the direction even more...Does the Starter crank the
Engine over each and every time you turn the Ignition Key to the Start position?
(In other words ,do you ever turn the key to Start and get no action from the
Starter???)
I'm starting to suspect the Electrical portion of the Ignition switch.Especially
with the lower Voltages you're showing at the GPs and the Starter.Of course
that could very well be a Bad Ground or the Dreaded Super High Resistance
Conductor.(Old Wire, 'Don't conduct so well anymore Burns Up lots of your Volts)

EDIT:

Even better...the NSS is Bad and when it does Semi-Function it ain't delivering the Full Voltage.
(Yeah,yeah, I like that hypothesis[If it's possible]
It'd be an easy FIX too... R+R the NSS... and be DRIVING!)

It's also come out that This Starter sometimes suffers the Heat Sink Blues
and the PO has just happened to remember having a Replacement that he
did not have time to replace.

No, it's not Jinxed. It's a Mercedes
(Without an RF Transmitter to call Stuttgart,for some much needed TLC)

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'84 300SD sold
124.128