A quick update. I am treating this as a high priority issue, although it is limited to the scenarios described above.
Unlike most edit screens this one has not transferred well to the new tab vs pop-up window change. So I have started to rework the entire construction, making it (I hope) simpler and more user-friendly.
I still hope to be able to supply an interim update here, but there is a possibility it will be too complex and need to be part of the next release instead.