Sorry but I don't think that will be possible - not flying the outbound effectively invalidates the return / inbound sector.

This is the same for both redemptions and paid for tickets.

The only exception is if you redeem miles and make two one way bookings (costs the same and provides greater flexibility). That way there is no tie between the outbound and inbound.

Certainly I wouldn't read anything from a past experience with Horizon Air as a forecast for success with this.

Something I'm not too sure of - is there any way you can cancel your current booking, retreive the airmiles and book again? Seems such a waste otherwise..

Good luck!