2-3-2 (LH 2.4) "Idle Long Term Fuel Trim Upper or Lower Limit": ECU mix control limits continually out of expected range during idle, either too rich or lean.

This should not be a restricted fuel or air delivery problem as that would be even further accentuated at run speed rather than just at idle. One would then expect a 1-1-3 code (short term upper/lower fuel trim limit at part load), 2-2-1 (long term upper fuel trim limit at part load) or 2-3-1 (long term upper/lower fuel trim limit at part load) code to be set. The fact these codes aren't present is probably significant just as you indicated. Consequently, I would not expect an in-tank pump problem -usually you'd get a noisy main pump and stumbling/stalling at speed with a partial tank of gas. That would also seem to rule out plugged air and gas filters. The injectors don't tend to fail, but with deposits they may leak or produce a poor spray pattern.

In addition to your items (and the FPR was certainly a good suspect),
o Check for unmetered intake air, i.e. vacuum leaks (esp. splits in the main air supply tube after the AMM, air leak around base of throttle body, brake booster valve). The unlit propane test may be helpful if you're careful. The LH system injector seals don't tend to leak, but you could still try spraying water at the base of each injector to check for an engine cough if it aspirates water.
o This could mean the start of a failing AMM -clean up the AMM electrical contacts and seal with dielectric grease and make sure the airbox pre-heat air flap thermostat is functional.
o You should note that the O2 sensor isn't used until the engine is fully warmed. As your code isn't being set for the first few miles then a faulty O2 sensor is possible. However, I'd expect it would have to be closer to a 5 mile warm-up rather than 20 miles for this theory to apply, however a faulty engine coolant temp sensor could allow that to happen.
o I'm mainly going with the fouled injector theory, especially a leaking injector. Rather than pulling the injectors for testing, I'd be very tempted (especially with the B234F which is prone to choking up with deposits from extended low rpm city driving) to feed it nothing but the best 92-94 octane gas for awhile and maybe to start a tankful or two with a quality, high potency, injector cleaner additive. Then take a good long run in the country hills keeping the revs well up over 3000 as much as you can. Clean/change the spark plugs after the cleaner additive has had a chance to flush out.

Ater that, I think a proper fuel rail pressure test is next.
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Dave -not to be confused with a real expert