If you want to get to the airport very early, there is UA 1210 that leaves SEA for ORD at 6:35. You might get there and try to go standby on that flight, which would likely give you the credit for the two separate legs.

The KVS tool is showing quite a bit of availability for the SEA-ORD leg that you are currently booked on. I'm not sure where you are seeing the 4 open seats, but if you are looking at seat maps, those are notoriously misleading.

If you were in a VDB situation, I think the best they would offer would be a certificate good for a free flight later, or a "$ off" coupon. I have never run across anyone offering a refund.

I think you could have some good success with the early standby to get you the full miles credit, and that is likely as good as you will do with this itinerary.