One thing which I think is important to keep in mind is that by the 1930s, many of the extraterritorial 19th century unequal treaties had been revoked already, in whole or in part, mostly in proportion to the damage the colonial power had suffered during the First World War. For example, the German and Austrian concessions were all revoked in the aftermath of WWI, albeit not without incident (frex, in Shandong), Russia's concessions were largely renounced by the USSR, most of the future Boxer reparations had been repatriated by the end of the 1920s, leaving only Britain, France, and Japan as major beneficiaries. Of these, only Japan had actively expanded her influence and extracted sizable concessions in the 1920s and 30s. Even among the British and French concessions, many of these were revoked during the 20s and 30s during the Northern Expedition, made possible by a combination of the relative stabilization of China and the utter incapability of an exhausted post-WWI Britain to object (something which did not apply to Japan). The comparison of the British acceptance of the KMT fait accompli following the occupation of Hankou to the heavy-handed Japanese military reactions to the Jinan and Shanghai incidents is illustrative.
With regards to historical withdrawals, I don't think that describing the French retrocession of Zhanjiang as an exchange is quite accurate; ROC troops never reached Indochina in WW2, and Chiang himself was not enthusiastic about the possibility. Historically, the retrocession of the various treaty concessions in China during WWII was essentially the result of a combination of American pressure and the mutual understanding between China and the remaining treaty powers (Britain and France, basically) that in the event of Allied victory, it was entirely within China's power to unilaterally occupy the extraterritorial settlements and revoke the extraterritoriality by force. IOW, it was better for the Allied powers to concede the settlements under relatively equal conditions rather than in a hypothetical post-war Allied victory, where China's position would be immeasurably stronger (of course, this is all moot in the event of an Axis victory). Hong Kong being retained itself is as much an accident of history and geography-despite greater British attachment, it is quite quite unlikely that HK would have been retained past the 50s had the KMT retained control of China in the late 40s.
Much of this can be credited to the effect of the Japanese occupation in effectively destroying the old colonial infrastructure in most of the Asian colonies. The loss of administrators, administrations, military bases, and military formations in the Far East as well as the depletion of the same in the metropole also contributed to the above change in the power balance between China and the colonial powers. The replacement by a (relatively) anti-colonial power*, the United States, also contributed to the lack of will and ability to maintain the old pre-war colonial systems. The total inability of the British, Dutch, and French to retain control of their larger colonial territories (Malaya, Indonesia, and Indochina) postwar is also illustrative of this phenomenon.
*Not totally, obviously, as America was also a colonial power in Asia pre-war, and retained semi-colonial relations post-war, but certainly, their enthusiasm for old European colonial empires was not great.
So where am I going with all of this? We can see from the above that the decolonization of the Chinese treaty ports basically had two proximate causes: the wholesale inability of the postwar colonial powers to defend outposts of limited value against a rising revanchist power (African and non-Chinese Asian decolonization also stems from this, but the lack of a revanchist power to enforce claims allowed the colonial administration to persist a while longer) following the devastation of the world wars, and in particular, the role of the Japanese in discrediting and destroying the old colonial administrators as a political force (this is not to say that Japan was not also seeking to become a colonial administrator, as she was, but rather that the defeat of both the old and the new colonial administrators left a situation where there was essentially no viable colonial administrator), allowing nationalist movements to take root. The second-tier outposts in China (inland outposts like Hankou and Jiujiang, specialized trade centers like Xiamen) were being steadily reduced already throughout the 20s and 30s by a resurgent China, with little apparent consequence, leaving only the prestige outposts, in this case, Zhanjiang, Hong Kong, Shanghai, and the Legation Quarter, plus the various Japanese concessions, such as Dairen, the rest of Shanghai, and the South Manchuria economic concessions (arguably, one could include Taiwan here as well, but Taiwan as an outright colony is not quite the same thing). In the event of a continuing Taisho era with no slide into militarism, I can't say I foresee the British or French keeping their concessions in the long-term regardless, with the exception of Hong Kong and Zhanjiang, the prestige colonies. Very likely, if the Great Depression continues, while the Nanjing Decade continues uninterrupted, the Shanghai, Boxer Protocol, and Legation Quarter extraterritorial concessions will gradually be abandoned, either by a combination of force or pressure on a part of a resurgent China. No Japanese militarism is, of course, almost certainly much better for China, as there will be no competing foreign policy to absorb resources that ought be dedicated to internal development, fighting the communists, disarming the warlords, and carrying out necessary economic and political reform. If there is a European war, but no corresponding Pacific war, the reabsorption will likely be accelerated. In this situation, Hong Kong and Zhanjiang will almost certainly be retroceded long before that war ends (if not, then the most likely results is that they are retroceded during decolonization-if there is no communist takeover, as is likely here, there will probably be no political barriers to retrocession on either side).
So this leaves the question of the Japanese concessions in Dairen and South Manchuria. One noteworthy factor is that without militarism, these probably will never evolve into the status of "full" colonies, as they did OTL, but rather remain economic concessions like the other extraterritorial concessions, the main difference between these and the European concessions that Japan retains the power to enforce her claims post-WWI, as most of the European powers do not. Hence, Japanese economic influence is likely to increase rather than decrease over the course of the 30s and 40s, if there is no war, as seems likely in this scenario. I do not know about the concessions being held past the late 40s and 50s, however. The general trends in China in the 30s was one of rapid economic development and modernization, which reduces the gap between Japan and China, especially if there is no war which at least initially, damages China much more than Japan. In this scenario, even with increased economic influence, the power to enforce claims on Japan's part declines respectively. Indeed, a slide towards either militarism or retrenchment (away from China) in this case seems inevitable then once the gap closes enough. As I noted above, in the face of a revanchist rising power, maintaining these sorts of extraterritorial claims is probably near impossible. The best example here may in fact be that of Japan in the late 19th and early 20th century, and her eventual re-negotiations of her various unequal treaties.
I do want to emphasize one more thing however-decolonization, at least in the form it took OTL, was not an inevitable factor. The relatively easy acquiescence of the colonial powers cannot be separated by the deeply traumatic national experiences of WWII, and the discrediting and weakening of the colonial powers as a result particularly relative to the United States. Without WWII, it is almost certain that the Algerian and Indochinese wars play out differently, as would Suez (very likely, Egypt would not be nearly in as strong a position as it was OTL here anyhow). In fact, in this situation, the likely result is every anti-colonial nationalist revolt playing out like an Algeria, except against a much less weakened colonial power. But this is rather tangential; as I noted, the situation is quite different between an anti-colonialist national revolt and a revanchist power asserting itself like China. This is merely to note that there is no reason to think that Chinese decolonization and African/Asian decolonization should take the same path, or that the ability of the European powers to retain their empires a bit longer in Africa implies the same wrt their extraterritorial concessions in China.