I think today is the first day I've seen any June or July DING! fares, so LAS/SEA is still a possibility. However, don't expect LAS DING! fares to be available for Friday or Sunday travel. If you're flying mid-week, and possibly Saturday, they may come up.

The prior advice to book the cheapest available now, as one-way flights, is solid. If a DING! comes available, using the two-step method you can cancel the existing reservation and use those funds for the new DING! Any funds not used are valid for travel for one year from original date of purchase.