Wasting SWUs Avoidable?
 

I bought an upgradeable DEN-BKK intl economy ticket. The first and last legs are short domestic flights, and those upgrades cleared. The international leg upgrades are still waitlisted, and I am now 3-1/2 days from departure.

It would be a shame to waste SWUs on the outgoing DEN-LAX and return SFO-DEN legs alone. However, when I do a first leg, I may not yet know whether any intl segment upgrades will clear.

If none of the intl upgrades clear, will UAL (on request) restore my SWUs and charge me some 500 milers for the short-haul domestic legs?

You might want to downgrade your domestic upgrade if your original booking class is still available, so that you do not run into wasting your SWU. I would recommend you do so every time you apply your SWU - you might need to call United to have an agent manually do it for you as it seems that the recent change in United website does not allow segment by segment upgrade. When your long haul is cleared, you can ask United to extend your SWU to the domestic portion.

IME the website never let you do do a segment-by-segment upgrade with any instrument except e500s.

You can call, and use the automated system to do segment-by-segment upgrade with any instrument.