If you have made the reservation for both of you for the Eurostar, then that reservation will be with you for both of you, and not with your wife. You wife's app will not know that you have made a reservation for her.

The reservations you make are separated from the pass, which is your ticket.

Is that on the Eurail website? Then just create a trip there - although normally one should have been created automatically after buying the pass.

Alternatively, you can book a reservation at a French ticket office or by phone via SNCF: press #85 for English, no booking fees, reservation is sent via e-mail.

My wife’s trip back to the UK is to all intents and purposes an entirely separate trip ( we both go to Belgium, but I have return earlier than she does). The website is incredibly slow. We have been tearing our hair out all afternoon trying to sort this out. 

Hello I hope that you've managed to make the single reservations back from Belgium. In case other travelers had the same question: you can simply tick the box of which traveler you want to make reservations for. After you've logged in, please navigate to seat reservations. A pop up should appear so that you can confirm for which trip and travelers you wish to make a reservation for. Simply select the appropriate travelers and continue with reserving your seats.