Not sure about the hot starting problem, but...

Are you sure the shifter was positively in either P or N? Sounds like on Wednesday it might have not been, so the P/N gear interlock switch prevented the starter from cranking. Or maybe your shift linkage needs new bushings and/or adjustment as it is no longer falling exactly in place on each gear position due to bushing wear.

When Mercs have trouble starting hot, one common cause may be related to air bubbles forming in the fuel lines (fuel cavitation I believe its called) because pressure is not maintained due to a faulty fuel accumulator or fuel pump(s). But if you've already replaced both, could be something else in your case. But just to be sure it might be a good idea to check the fuel system pressure with the engine running, and then see for how long a correct reading is maintained after the engine is shut off. I believe the accumulator should be able to keep the system pressurized for up to 30 minutes. It also now comes to mind that perhaps one of the injectors may be leaking fuel into a cylinder, or the cold start valve is leaking fuel into the intake manifold.

Good luck
-Alex

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W163 ML230 EURO SPEC 1999
W124 300E 1986