One way to Italy from the US

If you have proof of Italian residency I am sure that would suffice any questions upon entry into Italy without a return ticket. Not sure why Delta would care about a one way ticket. You are free to fly back on any airline.

Buy a return ticket that is cancellable and cancel it when in Italy.

Shouldn’t be a problem. Why don’t you ask Delta? I’ve flown to Venice on one airline and returned to the states on a different airline from Turin. Separate tickets. Also flew into Florence returning from Paris - different airlines, separate tickets never a problem. Two of the flights were on Delta.

No it's not a problem, you do NOT need a return ticket! That might cost $$ if cancelled. In the unlikely event you're asked at the airport, show your residency card, which is what I do. BTW, sometimes RT FROM Europe is much cheaper than from US so I buy my tickets in Sweden. I once used points to travel OW, then started buying from Sweden to US RT. (As long as you know approximately when you're returning to Italy, or understand the cancellation / change fees of your airline, which usually allow you to use ticket within a year.)

Unless you have a specific issue that causes you concern, use your USA passport and I don't think Delta or the Italian passport authorities will care. My wife and I flew from Seattle to Rome on Icelandair in October. We had one-way tickets and were never asked about our return plans. Have fun!

This came up recently, apparently there's a company that specifically sells you a fully refundable return ticket that you can show to the authorities if questioned.

Also, you can just buy a roundtrip which is often cheaper than a one way and not use the return portion.

This came up recently, apparently there's a company that specifically sells you a fully refundable return ticket that you can show to the authorities if questioned.

Not needed, if they have a residency card for Italy, a one way ticket is fine.

Also, you can just buy a roundtrip which is often cheaper than a one way and not use the return portion.

This is called "skiplagging", it violates the Rules of Carriage that you agree to in buying a ticket. The airline is within their rights to seek recovery of any cost difference (usually they do not), but they will penalize any status, including miles, that you have with the airline.

This is not a complicated issue, you show up at check-in, if they mention it, show your residence card (or extended visa), end of story.

Over the last 20 years or so we rarely if ever have a return ticket when we land. I think there is some urban myth around this question of having a return ticket. We have never been questioned about having a return ticket. Who has been question? Now we are retired and we don't look like we are going to over stay our visa. I really do not think you will have any problems.