Non rev can be precarious if there's not many flights and/or carriers on your route and the reality is that carriers will look after their own first.
As noted before, it's the passenger agent, well really the computer system, that decides. I have been helped by one agent who upgraded a few economy passengers to fit my family in the back.
You may have better chances taking a flight to Mexico City or Miami and finding a connection to Europe from there, but you may need to acquire more ZED tickets and turn in what you have for a refund.
In fact, I have on occasion traveled with two different return tickets in case my first choice was full; then refunded the unused ticket once back home.