Hmm... now we have to get into the discussion of whether archaic forms of a name are applicable. Not another canon argument!

Seriously though, I guess the real question is whether the name evolved from 'Aegthelion' to 'Ecthelion' in Middle-earth, or in Tolkien's writing. What did Tolkien intend for the name to be? And should we apply the evolution of a name outside Middle-earth to its evolution inside it?