Chiang's plan was to destroy the Communists once and for all. Only after that did he intend to go against Japan. He wanted a reasonably united China before trying to defy the Japanese.
Chiang went to Xi'an precisely because he was unhappy about the progress of eradicating the Communists. He clearly intended to take over the campaign and marshal his forces for one final encounter with the Communists.
What happens next really depends on whether you think Chiang would have succeeded against the Communists. He tended to do well against them when Mao was not in command, but not well if Mao was in charge. However, Yenan is their last stronghold, so the Communists might need to resist in open battle or risk losing everything. This is very debateable.
For sake of argument, let's say that Chiang succeeds and does so before the Marco Polo Bridge Incident in July 1937. What happens next? He's eliminated the main opposition to his rule outside the KMT, but there are still various warlords nominally in the KMT who oppose central control of China. Chiang's next steps are likely to begin asserting control in regions ruled by the most independent warlords, possibly provoking armed conflict. If so, Chiang will likely win. Some warlords cooperated with the KMT more than others, so some might be willing to fully cooperate under Chiang rather than risk defeatin rebellion.
Chiang will be under intense pressure to respond to the next Japanese incident, but he won't feel trapped by his promise made during the Xi'an Incident since he would never have made it. There will be intense pressure from the Chinese intelligentsia and students to respond. It's open to question what Chiang would do. I think he could probably survive if he decided to not respond.
Chiang's original plan was to wait several more years before he felt China was ready for war with Japan. Absent severe pressure, I think Chiang would prefer to stay with that plan and defer war until 1939/1940. His German advisor Falkenhausen had told Chiang in 1936 that he needed two more years before China was ready for war. Shortly after the Xi'an Incident, Falkenhausen optimistically revised that (he was clearly wrong), but in any case, Chiang can start beginning to defend coastal areas and prepare for a war in northern China, and not a retreat to Sichuan (which was the original plan in case of war, and what actually happened in the Sino-Japanese War).
If Chiang gets an extra two years, it will have immense repercussions. He will have more German trained divisions (for at least a total of 60 compared to 8 fully trained and another 12 partially trained by the time the Sino-Japanese War actually began) and a larger cadre of Whampoa officers. He will have more modern artillery and decent coastal defenses along with a German provided coastal navy of torpedo boats, coastal guns, and some larger ships (all planned to arrive by 1938). A more literate and effective central government. Increased tax collection. Increased Chinese light manufacturing and arsenal production. Sino-German economic plans in the 1936 Three Year Plan (which couldn't be fully implemented because of the outbreak of war in 1937) foresaw increased railroad building, major industrial zones in Hubei, Hunan, and Sichuan (all inland in China and away from Japanese troops), and the creation of an industrial technical class that would allow China to take over these concerns. China would have a solid basis to provide its own small arms, and have a standardized army.
While this is going on, Chiang even has a chance to institute radical reforms like land tenure and a central army payroll (which would destroy independent warlord power and give him complete control of the country's armed forces). Even if land reform and other reforms were only partially completed, it would make Chiang extremely popular with the people and strengthen the central government. Chiang was more progressive and enlightened than most warlords, so he is likely to to do something given the additional time, even if it is not completely revolutionary.
Internationally, the two year delay will mean German-Sino cooperation will make China - not Japan - Germany's preferred ally in East Asia. This will have interesting diplomatic repercussions.
If China can delay war until 1939, we will see a vastly different Sino-Japanese War. Chiang is likely to quickly seize Shanghai, and be able to defend the coasts. This means the war will take place in the Central China Plain north of the Yellow River. The Japanese are likely to not be able to advance very south of the Yellow, and will probably never threaten Wuhan or occupy Shanghai or Canton. Faced with a very hard war, Japan might very well agree to a negotiated peace after 1-2 years. Neither Japan nor China might become involved in World War II.
Of course this depends on a lot of things. If Chiang can quickly defeat the Communists in early 1937. If Chiang is able to prevent the Sino-Japanese War from breaking out in 1937. If Chiang is able to reduce the influence of the other warlords. If Chiang can prevent any war until 1939 or later. I think he has a good chance of doing this, but it is by no means for certain.
