I think the position should be clear. When in 1947 ROC published its claim to SCS EEZ didn't exist, wasn't even thought of, and territorial waters were 3 miles wide. It was clear since the English-Dutch wars of the 17th century that you can't claim open seas ( the English tried to claim exclusive fishing rights up to Dutch territorial waters! ) so the claim could only concern all islands within the cow tong with their territorial waters. We can then quarrel about the position of reefs that are or are not occasionally wholly covered by the sea.I agree that China is deliberately being ambiguous regarding its dashed map, however through their public announcements and reinforcements I don't think they've actually claimed the entire SCS within the line as their territorial waters, rather only the islands within the line. Unfortunately no one seems to have requested a clarification surrounding just what the line means.
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At the time the US, colonizers of the Philippines, France, colonizer of Indochina nor UK, colonizer of Malacca and North-West Borneo, protested and nor did Indonesia, independent but still defending itself against The Netherlands, that thought it needed to exploit its late colony to rebuild itself after WWII.
No-one seems to be interested in looking at the historical background and that is itself curious.