Mp3tag v3.02a adds various improvements regarding handling of action groups including most of the ones discussed in this topic.
You can now select multiple action groups and change their enabled state via a click on one of the checkmarks of the action groups in the selection (or via Space). You can also disable all action groups via the Ctrl+U keyboard shortcut or enable all via Ctrl+E.
I've added an indicator on how many action groups are selected to the action groups dialog.
You can now move items (also in groups) via keyboard shortcuts or drag'n'drop.