This sounds like a classic case of blocked fuel tank strainer. If this was a W124 the tank strainer is reached from under the car above the differential and unscrewed with a 46mm socket or a toilet wrench. It can be reused - just clean out the gunk from it with a good strong surfactant and hot water. I beleive this same job in the W210 is a pain as the strainer is a little to the side.

For the diesel leak, clean out the pump with simple green and water, then apply a hair dryer to it, take a light bulb of the highest luminosity you can get, start the engine and look for the leak. I have seen a post here showing the shutoff valve O ring going bad and leaking fuel slowly.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)