This one is hard to say because the 3.6 is junk. They will miss if they run low on oil. There were definitely some questionable decisions here, I'll run over each part one at a time.

For the first trip you had a p0306. On this engine 3 of the 6 coils can be reached very easily. 3 require pulling the plenum. Some shops are going to recommend replacing the 3 that require pulling the plenum at the same time. I recommend doing so on a Ford, because Ford coils are cheap and their failure rate is high. I wouldn't recommend it on a GM, but that they did so is not necessarily shady.

Second trip, if we knew the code here I could tell you a lot more. I'm going to guess that it was a p0300, because it is common. If the oil wasn't low, and there was any rattles, then it most likely needed a chain, these cars have an issue with them. You could have just been low on oil, though. We have several customers who bring them in for oil changes when they start missing because they refuse to check their own oil level.

Third trip. If there was really a chewed through wire, then they did this one. You said the light came back on the next day. Was it a 305 again?

A converter could have died from the misfire on 6 or 5. That is what kills converters, misfires. If we had been seeing misfires on a whole bank (Ie, cylinders 1,3,5 or 2,4,6 were misfiring, then we would suspect a converter.

If the only code you have right now is a p0305, then it is very doubtful that the converter is bad. If you have like a p0420, then a converter is probably dead, but it is doubtful it is causing a running issue. It is an efficiency code. Generally when a converter loses all of its guts, it won't throw an efficiency code, but it will make the car run like shit because it is causing a major exhaust blockage for 3 cylinders.

Info I need: Is the car running good? What is the oil level at? What is the new code? Have they checked for excessive carbon build up?