I bought a non-refundable round trip from the US to Europe in cash on Delta. I've now found a one-way US -> EU award ticket on another airline that I would like to buy, but would like to keep my return flight on Delta.

I paid about $1000 for the round trip ticket, but the return flight alone is about $1300 one way. What shocked me is that if I search for a round trip itinerary for EU -> US on the leg I want, and then a fake US -> EU leg I would no-show on, the itinerary works out to $1000 again (significantly less than the cost of the one-way!).

I tried searching but couldn't find many data points on how to deal with this.

Option 1) I cancel and get eCredits of $1000, pay ~$300 out of pocket for the one-way fare (I wouldn't want to hold the eCredit because i'm not sure I would have another use for them).
Option 2) I call Delta and see if I can change my round trip to a one-way, keeping the 2nd leg. It doesn't seem possible to do online. I'd be thrilled if I broke even on this. I can't find many examples of people doing this online, at least since change fees were eliminated. Would they price it based on the one way fare so I would still be out $300 out of pocket?
Option 3) I cancel and use the eCredits for the fake round trip itinerary, and no-show (or cancel in the middle?) on the second leg. I'm having trouble finding what the consequences of this would be.
Option 4) I book a round trip where i'd actually use the second leg. This isn't really going to be possible for personal reasons within whatever time frame is allowed.