it seems the right time, again, to repeat here one of my guerilla tips (they date back to the very early days of FlyerTalk):

Using only half a ticket or not using a last segment

I would, after having done that first one-way-segment, go to the airline and ask them to please annulate my return flight, as plans unfortunately changed and I might even have to take an earlier/or possibly later flight, and I don't want to be a no-show, so that they can resell my seat a second time.

I would politly ask for a refund of my return-ticket, as I will not use it. They will politly refuse to do so.

I would ask then, if they, at least, would please credit me the miles for the return flight that I paid for, but can't use. They will politly refuse to do so.

With tears in my eyes I would than leave ...