My return flight, I have will being heading out from Luxembourg using Air Dolomiti to Frankfurt, I have a international flight back to Chicago O'Hare, I have only an hour and 15 minute layover, the flight is united airlines, think this will give me enough time?

I can change my flight to Luxembourg to ZRH airport Swiss International flight which gives me over 2 hours which will be different when coming from Chicago to Luxembourg that will go through Frankfurt.

We've made a lot of connections in Frankfurt. That is a legal connection time. On a good day, it should work - but you never know if you'll hit Frankfurt on a good day or how far you'll have to walk. You'll have to go through immigration/passport control but not security. The passport control lines can be long depending on time of day and how many agents are on duty (I've been through a few times this year when there have only one or two agents working). Also, most of the Air Dolomiti flights I've been on have parked at a remote stand and used a bus to/from the terminal, so that could take some additional time. The United flight will start boarding 50 minutes before departure time. In general I prefer Zurich for connections.

I found that the flight I had picked for Zurich isn't available, I found one the next day that gave me a layover for an 1 hour and 35 minutes, is that reasonable?

It's 20min more reasonable.... personally I'd be comfortable with the Frankfurt connection. If the connection is super tight, Lufthansa ground staff you see upon arrival at the terminal can radio ahead to your Chicago flight and let them know you are coming. They may be able to whisk you through to your gate. Not sure exactly what they can/can't do but they are there for your help.

Air Dolomiti is a Lufthansa subsidiary, and thus a Star Alliance member along with United, so you likely are on one single itinerary / checked in/ bags checked...

I end changing to Zurich instead for my return instead, less risky of missing my flight. thanks everyone.