RESOLUTION! - perhaps...

In case anybody is still reading this(?), a bit more data may be of use to someone...

Tested fuel pressure at schraeder with proper gauge - it's about 50 psi and the pump maintains this pressure. Further, with key off, pressure does not bleed down appreciably, so it doesn't feel like any kind of injector leak (though re-sealing the injectors is on the Project Plan.)

No, it's worse - 'cause I've wasted a lot of time on this. As part of general new-purchase maintenance, I replaced the camshaft and crankshaft position sensors. Stepping back through/testing everything, I put the old (Mercedes-branded, fwiw) camshaft position sensor back in, and the car has started reliably ever since. Replacement was a Hella (made in China) part. Draw your own conclusions.

Problem (perhaps) solved. Fingers crossed.
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OK - still trying to diagnose...

Latest 'test': Car sitting for over a day; hadn't been started.

1) Attempt to start 'cold': Takes 2 'turnover' cycles to start. IE; position 2, engage starter, back to 0. Almost always starts on the 2nd try. 3rd at most.

2) Release of pressure at schraeder valve: yes; verified; some residual pressure in rail. Makes sense...

3) Holding key at position 2, definitely hear the whirring - I think? - of fuel pump, though hard to differentiate from SBC unit, which always seems to be pressurizing itself. Again, release of rail pressure produces a 'shot' of fuel.

However, car still starts immediately, even after release of rail pressure.
So, am I proving that I have a good fuel pump? Or that there may be some issue with pressure shutoff switch or sensor?

Again, car runs perfectly once running; no hesitations or evidence of fuel starvation under way. So it doesn't feel like an injector problem.

Thanks, Jim/others - Any experience/insights appreciated.

__________________
'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr