This is a silly discussion; all official Chinese maps before 1949 were made by the Republic of China government. Anyone could make maps and claims because China was not really united then, and it did have a free press, although there was nothing like QAnon.

The reason the PRC has made the claims so recently is because the PRC government felt that they were too weak to enforce those claims. Chinese political thinking dictates that claims which cannot be enforced should not be made because it will make the government weak in the eyes of its own people. Only when it feels strong enough to enforce the claim will it make it. The only exception to this rule is Taiwan because the island is too big, and because it became a Chinese province in 1887 under the Qing dynasty. Before 1887, it was administered as part of Fujian province. The other reason for the PRC claiming Taiwan is because after 1949, it became the seat of the ROC government, which also claimed to represent all of China and held China's seat at the UN until 1971, when it was replaced by the PRC.

Xi Jinping revived the claims to the SCS because he needed to show Chinese that the Chinese central government (CCP), that China was now strong enough to enforce its claims. The aim of this was to generate more China domestic support for him just as he was being installed in the main leadership role. I would say that he has succeeded with this aim, because by directly confronting the US, the leading superpower, he elevates China to the same rank and status as the US.

The survey ship used in the making of the nine-dot line claim was a US ship with KMT surveyors (around 1947). Nobody took the map seriously then because the ROC government was in the middle of losing a civil war with the Communists, and the Truman administration took a neutral stance between the Nationalists and Communists because George C. Marshall tried to negotiate a coalition government agreement between the Communists and Nationalists and failed. After the Communists came to power and the Korean War started, Joseph McCarthy started a fishing expedition to affix the blame as to "who lost China"? (As if the US had the power to determine who ruled China.)

The 1955 islands you mention were the Dachen Islands off Zhejiang province; US Navy ships helped in the withdrawal of ROC military and civilians to Taiwan: https://en.wikipedia.org/wiki/First_Taiwan_Strait_Crisis

My point in this is that if you look at China only through US eyes without understanding the historical background to the Chinese claim, you won't get anywhere in understanding US-China relations. Offering small chunks of history in order to sell a certain view without seeing the picture as a whole is dishonest.