No, for about six different reasons, including the fact that UA would then require the walk-up fare for IAD-SFO. It's against UA policy for them to drop segments in that manner unless there are IRROPS, to protect against hidden-city ticketing (as you later suggest).

It is rarely a good idea to escalate to a UA supervisor unless there is a technical issue that the front-line support agent can't resolve. I know it's painful advice after a 30-minute hold, but hanging up and calling again is almost always better than escalating for anything that involves policy, as the supervisor will support the agent 99.9% of the time.

In this case, it's not entirely clear that the agent is wrong. I mean, I think the agent is wrong, and WineCountryUA and Kacee both indicated that you can do this, and I'm 100% certain you can do it after you've taken the outbound. But I'm not sure that the agent isn't technically correct when you're trying to make this particular type of change prior to departure, especially if your ticket was fared as a round-trip. You may have gotten a different answer if you had said "I know I'm going to have to stay later than the rest of the party, but I'm not sure exactly how long; can you cancel my return now, to free up the seat, and then I'll call back when I have my travel plans finalized?" Just a thought for next time.

This will accomplish nothing. When you don't show up for the flight, you'll be offloaded and they'll put somebody else into your seat. The only people you'll affect will be gate agents who had nothing to do (personally, anyway with your issue. What I would do, however, now that you're booked onto Alaska, is to have your husband let you know if there's a flight delay on EWR-SFO. If there is, and it's substantial enough, call UA and ask them to refund your PNR.

There will be no impact on anyone else. UA plans for a certain number of no-shows. They're not going to hold a flight waiting for someone who checked in online but never showed up at the airport.

At T-15.

Precisely.

OP: As crazy as this sounds, UA (and most airlines) actually forbids buying a ticket and then not using it. I mean, they won't drag you onto the plane , but it's contrary to their contract of carriage (it's called throwaway ticketing). When you bought the ticket, they gave you a discount in exchange for some conditions; one of the conditions was that you would fly the itinerary as purchased or pay the necessary change fees to change it to something else. Now, they're known to be reasonable, and they understand that when the remaining credit is less than the change fee, few people are going to pay that fee, which is why I'm surprised that you ran into an agent who was being a stickler about it. But they're not wrong, per se -- they're just being pigheaded.