First, no one is going to get in trouble, UA understands plans change. UA will not send you a bill if you cancel your return flight. Not canceling, at all, will guarantee no credit (so, wait till day of travel). The worst that will happen to her is loss of E+ and reseated in E_(if you cancel too early). But waiting till day of travel to cancel, this minimizes the chance of loss of E+. But change her seat assignment now to the better of your two seats.

If you book travel on UA the same day as the other ticket, there could be an issue but that does not sounds like that will happen.

As far as using your flight credit, yes, the options are limited -- worth reading some prior posts for some ideas but your concerns on using the credit are well founded.