A faulty fuel pump check valve will not affect mix, and especially not idle mix. As you observed before, it will delay initial starting after the engine has been sitting as the fuel pressure in the fuel rail will have dropped and it will take a moment of cranking for the fuel pump to build pressure back up.

The LH 2.4 system should not allow the fuel pump to run unless the engine is turning over (either running or cranking during starting). Just turning on the ignition is not enough, the ECU must see the engine turning over as detected by the rpm sensor (aka. crank position sensor) on the bell housing (in the LH 2.2 systems it was the Hall effect sensor in the distributor). It's actually a safety design so fuel doesn't keep pumping and feeding an engine fire in case of an accident. The early LH 2.4 ECU's (primarily 1989 vintage) were prone to fuel pump control problems. It is possible that the previous owner, rather than replacing a faulty ECU, used a bypass that would operate the fuel pump relay directly from the ignition switch. Your year should not have the problematic ECU so I would not expect this has been done.

I have one other preventive maintenance fix that may affect your problem. The electrical connectors under the hood must be clean (use spray contact cleaner or isopropanol and a toothbrush) and should be sealed from moisture and corrosion with dielectric grease (don't use this on the O2 sensor connector -use OxGuard or similar). The AMM connector in particular is a known problem area. The block connectors under the right strut tower brace should also be opened, cleaned, greased and re-connected making sure the pins are fully connected by pushing them in from the back side -Volvo has issued TSB's on that topic.

After all you've replaced/checked and you've followed my suggestions, if your diagnostic code keeps appearing then I think it's time to swap in a known good AMM to see if that is the problem. A used one from a yard or eBay shouldn't cost you too much and a spare is always good to have. To help prevent AMM problems and failures, be sure to check/replace the warm air flap thermostatic piston in the air filter box or permanently close the warm air flap (see the FAQ).
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Dave -not to be confused with a real expert