Hi saguaros - sorry to send so many messages. I've resolved all of the issues relating to event field info.
The solutions to my questions (for others who are interested) were:
1) Only apply the fields to the subcategories, and not also to the parent category. This avoids rendering the field info twice.
2) You can change the date format directly in the field render.php file, without using the language override.
3) Make sure all fields are set to Show On Both in the field options tab.