I have a fairly similar situation to the OP when I fly next month.
I bought a Euro Traveller return, outbound Monday, return Wednesday.
I then had to delay my departure until Tuesday, but still want to return Wednesday. The cost of editing my booking was greater than buying a new single ticket for the Tuesday, so I bought that. (At this point in the lengthy phonecall, I said to the agent "that's more than a new single, so I should just buy a single." If I have made an error here, it would have been good of him to point it out.)
I have just realised that I am relying on a return leg that may be cancelled. Is my best hope to phone them now and ask them to protect the return? I'm not keen to cancel my outbound yet, just in case my Monday again becomes free and I can revert to Plan A.