Dear Keith,
about the issue with the fields that are available in the Notification settings if selected 'when custom fields are modified'.
The available fields that will be listed in the select box are fields of the types I've previously mentioned and that exist in the current form that your editing or creating.
Is the missing field listed in the form?
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Ana