I can't help you with the second part of the question, but I can help with the first bit:

Chiang Kai-shek was extremely suspicious of foreign influences in China. If there's any one theme I could characterize to Chiang, it was an almost paranoid fear that foreign powers if given the chance would attempt to snatch pieces of a weak China and turn them into puppet regimes. For instance, he had been involved in a low-intensity guerilla war in Xinjiang between Soviet-backed rebels and the local Muslim warlords of Qinghai, sending in supplies and arms to the rebels (one notable incident, for instance, was a Muslim cavalry patrol capturing a supply caravan, only to discover that said supply caravan had been meant to resupply their unit in the first place). Tibet was no exception to this rule. He had fears that both Britain and the Soviet Union were interested in breaking Tibet out of China to be turned into a puppet buffer state. The British would use Tibet as a buffer to protect British India from either China and/or the Soviets, the Soviets would gain it with some sort of partition with the Japanese. He even claimed that Japanese officers had "naively" attempted to get the Soviets to help the Japanese partition China in the midst of the Second Sino-Japanese War.

In terms of actual action, Chiang, who was mostly preoccupied with the struggle with the Japanese, didn't have the troops, resources, or political capital necessary to squash any budding independence movement, despite Tibet making the declaration that it was an independent entity and making some diplomatic overtures on its own. What he did do was give the Muslim warlords free reign to crush Tibetean rebels in Qinghai as well as to make preparations to invade Tibet should any decisive action be taken. The Muslim warlords were very loyal to Chiang, their cavalry being some of the most elite troops in the entire National Revolutionary Army, and promptly did so. Later on, some of these same Muslim cavalry ended up participating in the PLA invasion of Tibet, after they submitted to the Communists.

Source: Soviet Russia in China, Chiang Kai-Shek