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Yes, I'd also think hard about the EGR valve being at least part of your problem.
A failed EGR valve can cause very rich running, meaning a poor idle, sooty plugs, smoke out the exhaust and notably poor mileage. The low tech approach is to suck on the vacuum line and you should be able to see the diaphragm move or else stick your fingers in to check. If it's stuck open, consider temporarily inserting a blocking plate in the intake manifold flange to disable it and see if that helps (after re-cleaning the plugs).
Yes, consider swapping in another ECU for testing, especially if it's from an EGR engine. ECUs are sometimes different for EGR and non-EGR engines. Some ECUs reportedly work with both, which I suspect is mostly EGR ECUs working well with a non-EGR engine. What's the Bosch part number on the current ECU? (Bosch p/n starts 0280). A Google search can find you a few ECU lookup tables to see if it's noted as EGR or non-EGR. Note that Bill Garland's ECU tables, provided to him by user ZJZ here, came from a variety of official and unofficial sources, so try to find one by Volvo or Bosch.
Is there a cold start injector? If that was leaking you could easily have those symptoms. I thought they always used the EGR port on the back of the intake manifold, so I expect you don't have one. ECUs are designed to either work with or not work with a cold start injector.
Another classic candidate for rich running is a faulty FPR, as you've already tried to test. The FAQ here has a fair bit of info.
If you haven't found it yet, even better is to refer to the Volvo 240 LH 2.4 green Service Manual (1989-on) which can be found here in the OzVolvo technical archives along with numerous other manuals
https://ozvolvo.org/archive/?download=VFAzMTM2MS0xX0xILUpldF8yLjRfQjIzMC5wZGY=
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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