Does my luggage have a chance of making it?

You should be able to check you luggage through to your final destination but if you have any concerns you could double check with United. As long as your luggage is checked through the 1.5 hours should be enough time. You should be prepared for passport checks on both departure and arrival in both directions and customs only on arrival. In my experience there is no actual customs, as opposed to immigration, inspection on arrival in Europe. As long as you have nothing to declare you simply walk out the door.

I'll answer them one question at a time:

1. Likely yes you can interline it. Call the airline and have them link your itineraries if you haven't already.
2. No problem with the bag making the flight. 1.5 hours is MORE than enough time to make a connection. 3. If you've never connected through O'Hare though, what you will have to do after the get off your Untied Flight is go out of security and follow the signs to the Airport Transit System (ATS). You'll take it to Terminal 5 (you'll arrive at Terminal 1 or 2) where you go through security again.
3. Yes you'll have to go through Italian customs when you first enter Italy. However, as you'll be connecting to an Italian domestic flight in Rome, I don't think you have to claim your bags as they will be transfered.

A lot may depend on how much time you have between you flights at Chicago. We flew United to LAX for our transfer to Lufthansa and we had about 2.5hrs between flights. We decided to not check our luggage through and retrieved it. We then walked over to the International building and checked in with Lufthansa there. This scenario worked great for our situation.