To be honest Stephanie, I can see no problem in what you are doing. You can for a reasonable price, obtain a casket from the funeral directors, making sure there is no metalic bits on the casket, and pack it into your suitcase, undetected, and take them to Corfu. Friends of mine did this, however, they drove to Corfu, and the ferry controls are not as strict as the airports.
If you just wanted to take a portion of the ashes, it would obviously be easier than all the red tape that is need.
Have you had a chat with the funeral directors on this. Just to find out what they say.