[oil pan 4]

The minimum rated RPM and oil pressure just depends on each individual engine. How much clearance there is between crank, rod and cam bearings.

I think the minimum RPM for lugging the engine is whatever cummins recommends for the minimum oil pressure and at the speed that does not give you that resonance vibration. That vibration will just tear up motor mounts, clutch in a manual transmissions, fatigue any metal line allowing it to break.

Don't worry about spinning the accessories too slow, those are easy fixes that can be fixed more than one way.

I don't think you can supplement the engine oil very well. I would bet that the oil pump moves 3 or 4 GPM at idle and about 30GMP at full speed.
The only electric oil pump I have found that might be used is only rated from 2.5 to 3.5 GMP. That may or may not be enough to supply the engine under load.

Another way to increase oil pressure and spool the turbo faster is run an oil restrictor to the turbo.
I ran a .030'' restrictor to my holset HE351VE for over a year until a piece of dirt plugged the nozzle. I punched the restrictor out to 0.050'', doubling the flow, added an inline oil filter and I hooked up the coolant lines.
Have not had any problems since.
I read the holset manual for turbo oiling, the 0.030 restrictor did not give quite enough oil to the turbo per their recommendations but it ran fine. I pulled the return line off the engine and put it in a jug and ran the engine to measure oil flow through the turbo.