That's one possibility. I have been told at times that one can't cross the same location twice (stopover or transit) so you need to think carefully on this.
Removing LHR-DUB should help and with that new routing you should be able to get the agent to send it to ticketing team for a look again, though the concern here is either the agent will take NAN is the splitting point between outbound/inbound, and then whether it was possible to have further open-jaw on the inbound segment that way; or will the agent realize the actual splitting point should have been TFU and then the routing makes no sense (too indirect) and reject it once again. Good luck!