Perhaps the easiest way to deal with all these potential costs is for airlines to calculate the cost of compensation and allowing the use of return portions of tickets only and thereby create a new compensation surcharge and add it to the ticket price.
This is clear and transparent and passengers will be informed of the additional cost of EU legislation.

BTW I am amused by the using of
the return portion of a ticket if the outbound is not used, as by definition if you have not flown the outbound, you will still be at your place of origin.