Before EASA you used to be able to do the rating in an ICAO country and return to Europe and take the Seaplane exams and apply for the rating on your licence. After EAS that all canned and now you can only do it through an EASA ATO. There are very few of us in Europe who can do the Flight Test and writen exams for you now. I am surprised you have had an issue with Como, they are usually excellent. Biscarrosse are also superb, I have done skill tests for them on occasion. There used to be an Instructor there that spoke good enough English to teach the course and that with the UK seaplane exams in English is enough to get the rating done. There is also a school in Norway that does it but restricted to the summer.