To book a 1-way with CO miles costs RT mileage (25K)

& must be phoned in--not online. U would need to use 25K more to get back on a separate 1-way--forgeddaboudit!
Just grab your desired outbound & an undesirable return flight. If a better flight opens up later, go to an airport to try to get an employee to change it without the change fee. If no-go, surrender to the change fee.
IMHO, $350 ain't bad for this flight.