Yep, IAC could be acting as a vacuum leak causing the low RPM stumbling and hunting. Low fuel mileage and stumbling at low rpm/at startup could also be a plugged up fuel filter that the fuel pump takes a while to build pressure through. I suppose a failing FIAV can also appear as a vacuum leak, but that seems less likely in your case (stumbling while both warm and cold idle).

To check if the IAC is implicated, you can steer lock to lock while parked and watch how RPM reacts, it should increase matching the smoothness of your steering (steer quickly, RPM should rise quickly, steer slowly, RPM should rise slower). I remember you had AC removed, but steering exertion should also trigger the IAC to compensate for increased load. If you have a multimeter, you can check the resistance of the coils and measure the switching frequency during operation (iirc, 25-35 counts at idle, 65+ counts with AC/steering on). To test the IAC while warm, you can take it out, warm it up with a hair drier/heat gun (be conservative), and then test the coil resistance again.