I think the real question mark is related to 2” radius and where is has to be met. My opinion is that the rule makes it clear: “the crown on top of seat must be a minimum of 2’’(50.8 mm) radius”. There is only one top of the crown and it doesn’t matter if seat back is 11” or more. Top is top. There is no word about area.
Therefore I believe below (attached) drawing represents my take on A.13.
Green = compliant
Red = illegal
http://http://serwer1376810.home.pl/P55_Images/backseat.jpg
the discussion about how it influences boat performance and is non relevant for this case.
I agree to 1-4 mentioned by Jeff.
