SEA/YVR to BOS/LGA/JFK does not scream UA to me to be honest. That said, I was able to book BOS-SEA r/t for $750 in P-class and that can be routed via SFO. So if time is not that much of an issue if can work.
There are some interesting routes ex-YVR on AC ... not sure AC brings back the YVR-BOS pilot again post COVID.