In history, the Communist Party of China advocated the release of Chiang Kai-shek because they believed that Chiang Kai-shek was a pro-British-American faction, at least from the perspective of anti-Japanese, better than the pro-Japanese faction such as He Yingqin.
Actually, they advocated his release
because Stalin told them to, for the reasons you mentioned. (OK, technically because the Comintern told them to.) Mao's original reaction to the "Incident" was one of delight: Chiang owed them a "blood debt" dating back from 1927, and must be put on trial...
https://books.google.com/books?id=qFmn6gdMlsUC&pg=PA108
To quote an old post of mine:
***
The best POD for a pro-Japanese government of China would be for Chiang Kai-shek to be killed during the Xi'an Incident--which came close to happening:
"Just before the attack at Lintong, Zhuang Xueliang had cabled Mao that he was about to act. Mao told his secretary, 'There will be good news in the morning.' At noon the next day a radioman rushed into Mao's cave and handed him an urgent message from Zhang Xueliang. One by one the CCP leaders hurried into the leader's primitive headquarters to hear the news. When Mao read out the news, the cave echoed with excited laughter and gleeful voices. Zhu De, Zhang Guotao, and others wanted to see Chiang and his fellow KMT generals killed immediately. Mao, 'laughing like mad,' felt the same way. Nonetheless, he immediately sought guidance from Moscow, proposing that Chiang be delivered for trial by 'the people.' Then the Chairman sent obsequious messages to the Young Marshal, calling him the 'national leader in resisting Japan,' extolling his 'world-shaking moves,' and hinting that he should deal with Chiang 'resolutely.'
"News of the kidnapping reached Moscow a few hours later, but unlike Mao, Stalin did not laugh; instead, he immediately saw that the event could be disastrous for the Soviet Union. The next day the Comintern received Chen Lifu's message and very likely read reports that He Yingqin had ordered Central Army divisions--probably the elite units--to move toward Xi'an and also had urged Wang Jingwei to rush home [from Europe--where incidentally he met with Hitler and discussed China joining the Anti-Comintern Pact-- DT]. The possibility suddenly loomed that the Generalissimo would be killed and Wang and He would establish a pro-Japanese government. Stalin sent a flash message to Mao telling him in no uncertain terms that the Soviet Union disapproved of the 'plot'--and suggesting that it was being staged by the Japanese. He ordered Mao to hold friendly talks with Chiang, find a peaceful solution, and release the KMT leader. In response to Stalin's orders, on December 15 a public telegram signed by Mao, Zhou [Enlai], and Zhu announced that the CCP stood for a peaceful solution of the 'Xi'an incident' and that any hasty moves would 'only delight the Japanese.'..."
https://books.google.com/books?id=DUg2KGMQWHQC&pg=PA129
