Do truly air-sealed electrically-actuated duct damper exist

It depends on a few factors but just based off of what you've mentioned I'd say that it probably would be fairly good at keeping the room a more moderate temperature.

This is mostly because the air handling unit will be drawing the hot air out of your room and in the process the cooler air from the rest of the house will displace the hot air. Since you mentioned that the room cools very quickly I'd venture a guess and say that the room has good air flow in and out (air will flow down the path of least resistance) or is very close to the air handler (making the installation of a return simple).

It depends a lot on how easy and/or costly it is to install an air return in the room but it seems like a fairly good idea. The new California residential building codes are actually mandating a whole house vent fan to effectively accomplish the same thing (cooling without needing to run the AC) because it's really effective at reducing the temperature more efficiently.

If you do move forward with installing air return remember that the higher the intake is the hotter the air it draws in will be.

It is difficult to properly cool two spaces with vastly different heat loads and only one thermostat.

Assuming typical computers and monitors and two users, you probably have about a kilowatt of heating or 3400 BTUs per hour.

Rather than run ducts, have you considered a dedicated cooling unit for your office? You can go with a small window unit. Even the smallest one is 5000 BTU and only uses about 500 watts when running. That's just a few pennies per hour and even that is offset by the reduction in load on the main unit. With the thermostat in the same room, its load response will be much better and the temperature extremes you have been suffering with will be eliminated. Plus you can turn it off when no one is there and save energy that way.

Lately my thinking is that ducts are basically parasites and efficiency goes up when the heater/cooler is in the same room as the load. In the past, the equipment cost was lower with a shared unit but operating costs (energy) have gone up a lot (and will continue to go up) and that could change the equation on selecting shared or dedicated equipment. Comfort will always be superior with dedicated equipment.