Thanks, Florian. That will be useful. I still wish there was a way to execute single actions from within the Action groups dialog, as that would be the way I'd use it most often.
Also, I think most people might prefer that they be shown in the same order they appear in the dialog box. The order in which they're placed within the Action groups dialog is significant, and seeing them in the familiar order would make using the drop-down easier.
I'm afraid, though, that I'm stuck at using Mp3tag 2.39f until the issue with checkboxes is addressed. That seemingly small change to the Options was a killer for me.