Weird 116 No-Start

[Strife]

85 380SL, 120K miles, good and recent tune

It just dropped dead after being parked for four hours and wouldn't start. Here's what I did:

1. Checked wiring at fuel pump, checked motor of fuel pump, pump spins
2. Substituted FP relay with "probably good" one, no result
3. Jumpered pins at FP relay (30/87), FP operates
4. I can hear the FP run after failed start cranking for about 1 second with the relay in.
5. I know I am getting a spark, verified with timing light.

Questions:

My 86 560SL FP operates in RUN even with the engine not running, I can hear it. Does the 85 operate in START (crank signal?) or RPM signal ONLY? BTW, I know that the FP relay is getting power at terminal 15 (fused, hot in START or RUN) and at 30 (always). The 85 doesn't run the motor in RUN when you first turn the key, and I don't remember if it did or not.

Can a fuel pump go "kinda bad", where it pumps but doesn't deliver the fuel? I have pressure gauges if you think this is worthwhile. This pump is not a Bosch (yeah, I know, a mistake) and it has 20K miles on it.

My next step is to substitute the fuel pump (with a Bosch). An oddity, I have spare 560SL fuel pumps, which look the same but have different part numbers, and the resistance of the 560SL pump motors is noticeably higher than the installed 380 pump. Maybe this pump is drawing too much power and pumping slowly?

This is driving me crazy, because an "easy" problem, which I was prepared for, would have been found by now!