Salin turned out to be the last battle of the war. After the successive battlefield defeats, the bickering between the Confederation's two main powers broke out in full force. Sawlon II of Mohnyin, who had only grudgingly agreed to Hkonmaing's takeover of the Ava throne, blamed Hkonmaing for the defeats, and now planned to put Sithu Kyawhtin I of Salin on the Ava throne. In April/May 1545, Sawlon II sent Sithu Kyawhtin with an army , which went on to occupy Sagaing, the city directly across the Irrawaddy from Ava. During the rainy season, c. September 1545, Hkonmaing died, and was succeeded by his son Narapati III. The new king promptly sent a mission to Pegu to secure friendly relations in exchange for his recognition of the new de facto border between the two kingdoms. Tabinshwehti accepted the offer. Toungoo's decisive victory gave the upstart kingdom control of all of central Burma, and cemented its emergence as the largest polity in Burma since the fall of Pagan Empire in 1287. Indeed, "there was once more a king in Burma". In the following years, Ava and Mohnyin-backed Sagaing would be locked in a war until 1551, while an emboldened Toungoo would turn its attention to conquering Arakan in 1545-47, and Siam in 1547-49.

What happened first: Sawlon II sent Sithu Kyawhtin or fall of Pagan Empire?
fall of Pagan Empire
In order to solve the question "What happened first: Sawlon II sent Sithu Kyawhtin or fall of Pagan Empire?", we need to check when each event occurred. For Sagaing, it is mentioned that he was sent in 1545 and occupied Sagaing city directly across the Irrawaddy from Ava. Meanwhile for Fall of Pagan Empire, it is said that Toungoo's decisive victory gave upstart kingdom control of all central Burma since 1287 (which means after 1287). Since before 1287 came first than 1545, then The fall of pagan empire occured before Sawlon II sendt Sithu kyawhin.