I've read that 1927-1937 was a period of consolidation for Chiang's KMT during which there was no external conflict (Japan's 1931 annexation of Manchuria did not develop into a full-blown war) and relatively weak internal resistance.
Did any of the concessions from the Qing-era Unequal Treaties get reversed/moderated during that time? Was Chinese sovereignty reestablished in certain areas?
To add on to it, were there any concessions reversed/moderated or sovereignty restored a) post-WWI and b) post-Xinhai revolution?
The answer to your question is yes.
The Unequal Treaties concerning Germany and Austria were reversed in the aftermath of World War I, as the Republic of China supported the Allies. For example, China succeeded in revoking the shares of the Boxer indemnities to Germany and Austria during the Paris Peace Conference. However, the Paris Peace Conference did not restore Chinese rule over Qingdao, which was given to Japan, which caused great anger and resentment within China.
Between 1908 and 1927, the Chinese succeeded in revoking the majority of the Boxer Indemnity payments and used that money to build infrastructure within China. It began in 1908 when the US agreed to remit a portion of the money to support the education of Chinese students in America and to build Tsinghua University. In 1924, the US remitted the rest of the money back to China. The USSR, Great Britain, France, the Netherlands, Belgium, Italy, and finally Japan followed suit.
Chiang Kai-shek did want to renegotiate the Unequal Treaties on equal terms and he made significant progress in the Nanjing Decade. In the late 1920s and the 1930s, the Chinese took back many concessions from the Europeans: the Hankou and Jinjiang British Concessions in 1927, the Weihai British Concession in 1930, and in 1931 took back concessions owned by Belgium. The new ROC government regained sovereign control over custom duties, got rid of the "most favored nation" status to many European countries, and China negotiated with Western powers to gradually end extraterritoriality. The war with Japan forced Chiang to postpone further efforts in negotiating the treaties.
As China was a major ally against Japan, both the US and the UK voluntarily gave up extraterritoriality rights in 1943. Japan and Italy lost theirs as a result of the war, and the French gave up extraterritoriality in 1946. The Chinese Civil War followed right after and so renegotiation was once again postponed.
Though the new Communist government refused to recognize the Unequal Treaties, they did make major exceptions in Hong Kong and Macau. To keep good ties with the Soviet Union, they also reaffirmed the old border treaties with the Russian Empire which ceded outer Manchuria to the Russians and set the Northeastern border.