UPDATE #2:
Connected a scanner tool and nothing showed up. The Innova scanner detected nothing: no current or stored codes, and nothing with the Ford specified scan.
Cleaned the MAF sensor (dirty) and throttle body (heavy carbon build-up) - did not fix issue.
Going back to when the spark plugs were replaced, there was no improvement. A few hours passed and the engine drove really rough. Took it back to the mechanic who did the plugs and he said there was a bad fuel injector. He replaced it and the roughness of the engine improved, but was similar to before he replaced the plugs. Engine is still pretty rough with a rough idle and hesitation. After taking the car to replace the plugs and bad injector, driving it at highway speeds now feels extremely rough in terms of smoothness of ride, similar to over-inflated tires and a stiff suspension.
As of now, I'm planning to replace the remaining fuel injectors this weekend (and possibly fuel pressure test) and go from there.
Also: I had the catalytic converters replaced a while ago and the mechanic (different mechanic) made a poor connection from the new converter to the manifold, which has resulted over time into pretty loud and smelly exhaust leak - sounds like a go-kart. Is there a possibility that this exhaust leak is also causing the engine shuttering, etc. symptoms?