Have you,in your diagnostic frenzies,Cleaned and Polished ALL of your Electrical
Connections? (Grounds[including the Ground Strap from the Engine to the Frame],Etc,Etc.)
'SubSet of the above:Have you Ohmed out your cables?
How is your OVP Relay? (Have you used the FSM test sequence?)

Is there a test sequence for your Glow Plug "PreGlow Relay"?
I ask that because something is almost instantaneously draining your Battery's
power...and the Culprit may be the GP relay.
The time delay in the system showing 12+ volts may be related to the GP Relay's delay in releasing the system.

Or the whole mess could be caused by a bad Regulator,helped along by a bad
connection between the Ignition Switch,Alternator Light(Dash) and the Regulator.

Or let's look at the other possibility ...BAD STARTER.

Still doesn't explain the SLOW increase of Voltage after start.

So I'm gonna go with the starting problems caused by your Starter's excess
need for Electrons.(Has it been on there for 300K?)
And the LAZY increase in Voltage after Start being the GP Relay.

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'84 300SD sold
124.128