I've never used import in Composr, but one thing that I always find helpful in other systems was the ability to delete matched entries on import. If you are going to treat matched entries as edits, you might want to offer this feature as well.

Test set...

CSV import: Meta keywords may be filled if we choose which field to use and make an import with new records
CSV import: If specified keywords field does not exist in CSV, error given
CSV import: Meta description may be filled if we choose which field to use and make an import with new records
CSV import: If specified meta description field does not exist in CSV, error given
CSV import: Notes may be filled if we choose which field to use and make an import with new records
CSV import: If specified notes field does not exist in CSV, error given
CSV import: Imports all records if no meta keywords, no meta description, no notes, no key field given
CSV import: The 'allow comments' setting transfers over correctly if ticked (checked)
CSV import: The 'allow comments' setting transfers over correctly if not ticked (checked)
CSV import: Key-specific settings are locked unless a key field is entered
CSV import: If specified key field does not exist in CSV, error given
CSV import: Exporting a CSV from a catalogue then reimporting, with the ID field as 'ID', makes no change to the number of entries in the catalogue
CSV import: With the key field set to 'ID' and 'Handling of new records' set to 'Skip', a blank catalogue will stay blank
CSV import: With the key field set to 'ID' and 'Handling of new records' set to 'Add', the number of records in a blank catalogue will then be equal to one less than the number of rows in the CSV
CSV import: With the key field set to 'ID' and 'Handling of unmatched existing records' set to 'Leave alone', an empty CSV imported into a non-blank catalogue will make no change to the contents of the catalogue
CSV import: With the key field set to 'ID' and 'Handling of unmatched existing records' set to 'Delete', an empty CSV imported into a non-blank catalogue will erase the contents of the catalogue
CSV import: With the key field set to 'ID' and 'Handing of matched records' set to 'Skip', an imported exported CSV with a column blanked out, will result in no changes
CSV import: With the key field set to 'ID' and 'Handing of matched records' set to 'Overwrite only fields which aren't blank/missing in the CSV', an imported exported CSV with a column blanked out, will result in no changes
CSV import: With the key field set to 'ID' and 'Handing of matched records' set to 'Delete', an imported exported CSV, will result in an empty catalogue
CSV import: the key field set to some non-ID column title, a CSV will import correctly binding according to that as a key field

csv.zip contains the implemented changes, compatible with version 8.