It would be very helpful in establishing a baseline for arguments/debates/compromises (or no compromises) if China files papers with the UN and clearly define what it claims in the SCS, and what exactly the the 9-dash line means.
I think that would provide too much insight into strategic planning and only serve to highlight internal Chinese priorities & concerns. Very much in opposition to Sun Tzu philosophies.
My take is that anything China does, particularly in the SCS & ECS, will be viewed critically and labelled 'Aggressive' or 'Potentially Destabilizing' by the US & Western media outlets and the Chinese know it. Those are the buzz words in most reporting on China these days. If China were to delineate it's interests that would only focus the criticism and clarify it's plans and potential weaknesses to it's competitors regionally & abroad.
Better for China to manage the blanket criticism, which they can shrug off as the standard 'Rising China Threat' rhetoric, rather than be forced into a corner on specific strategic plans which it they then must defend in detail -- providing answers which will never be satisfactory to those asking the questions in the first place. All part of the game.