Honda Accord Euro Engine Problems
Euro break down
Unfortunately it would appear you are dealing with an incompetent or deceptive dealer. Ask to see proof of what work has been done by the dealer, and have them justify their findings to you. I would also take the case to Honda direct and demand head office intervention. Also consider taking your case to consumer affairs.
Honda Accord CO2 sensor
Honda claimed an overall average of 9.2 L/100 km, but that's derived from a lab test and not real life. Even so it should be an indication of what you could expect, and that seems to be mirrored by your real life results. It would seem that it's probably running rich and your theory of a faulty CO2 sensor could be on the money. Your dealer should be able to check it and confirm if it's a problem or not.