As stated in the OP, entering the PNR on Lufthansa.com returns the error ""Your booking can only be serviced if the flight tickets are issued and the payment is confirmed. Please contact your original booking office or your Service Center for more information."

I received a form reply back from Eurowings, stating "After a thorough check, we were unable to find an active booking with your details.So that we can deepen our efforts, we would like to ask you for further information.Please provide us with the flight date, route, flight number and full passenger names.", notwithstanding that I had entered the PNR and provided all of that information in a file attached to the web contact form.

The form reply also said to contact the other airline if the booking was made through another airline. I am afraid that UA will tell me I need to contact Eurowings, and Eurowings will tell me I need to contact UA. But I'll try to call UA.