Phil:
I shared your story with an aquaintence of mine today. He is a recently retired FAA inspector and agreed with my assessment of your situation and what you must do under the regulations to upgrade your SEL rating to ATP privileges. He said if you came to him, he would have given you the ATP SEL checkride and issued the certificate if you passed. When I mentioned that you had gotten different answers from several different sources, he smiled and said he was not surprised! He saw this alot in his 35 year career. His suggestion was simply to find an examiner who agrees that (s)he can give you the checkride. I agree. Not all examiners or inspectors interpret the rules uniformly. Some discretion will sometimes be exersized when rules appear to be contradictory or ambiguous.
In your case, as the holder of an ATP certificate, you only seek to add a class of aircraft within the airplane category to your ATP privileges. Since you are allready an ATP, you do not need a knowledge test, any required training or an endorsement from a CFI. Just a checkride. The rules are right there to read. It's just a matter of finding an examiner who understands and interprets the rules in this way.
I suggest contacting some examiners directly by phone and presenting your situation to them. Seaplane examiners probably encounter this situation more often than most other examiners since they do so many additional class rating flight tests. When you find one who will agree to give the ATP SEL checkride as an additional class rating on your existing ATP certificate, make an appointment for next month. Obtain a current PTS and assure that you are flight and knowledge proficient in all required areas of operation. Even though it is not required, it's still a good idea to get an objective assessment (practice checkride) from a senior CFI as soon as you feel you are ready to begin your preparations for the test. Then you will have a better idea how much study and practice would be prudent before proceeding with the test.
Here is a
LINK to the FAA designee locator page. Search for ATP examiners in your area by selecting ATPE and the FSDO office in the area you desire. The search will return names, addresses and phone numbers.See what you can find out that way.
Best of luck,
Westhawk