In some other lighting software I have used, the approach to this is issue is pretty straight forward and basically the same behaviour that you would see in simple desk.
If you choose to over-ride a channel then that value persists until you clear the change.
My recommendation would be to have a slider that monitors the output of the channel and then if the operator chooses to adjust that level the slider turns red, as in done in the simple desk, and then has a button that clears the change and returns the channel to the level specified by the running cue. (I guess a fade could be incorporated here)
And then... a really nice feature would be able to save that level to the active scene (maybe with a button on the slider), effectively updating the scene. In my (limited) experience of doing lighting for theatre shows, especially during dress rehearsals and pre-peformances, the ability to make a small change and then save it to the scene for the next performance is really useful. This can be done in live edit I know, but sometimes that takes too long. At the moment, I scribble down my changes and then have to remember make the change to the scene at the end of the show. Relying on my memory or scribble notes is not a good thing.
I have no idea how hard this is to program and I really appreciate the people with the talent and the know-how who have created a really great bit of lighting software.