Short answers, pulled from the story.
Prince Mongkut entered the Buddhist order in 1824 with the ordination name Vajirayan. He spent twenty-seven years as a monk before ascending to the throne in 1851 at age forty-seven.
King Mongkut began a reform movement in 1835 after meeting monk Buddhawangso who strictly followed monastic laws. This initiative created the Dhammayuttika Nikaya sect which emphasized spiritual affairs over worldly matters.
King Mongkut hired English and American missionaries in 1852 to teach princes modern subjects like geography and astronomy. He also ordered nobles to wear shirts when attending court to present Siam as a modern nation from a Western perspective.
The Bowring Treaty abolished the Royal Storage monopoly that had controlled foreign trade since Ayutthaya times. It reduced import taxation to three percent collectible once per transaction and granted extraterritoriality allowing British subjects to be judged under British law.
King Mongkut never offered war elephants to US President Abraham Lincoln but sent domesticated elephants to President James Buchanan via a royal letter dated the 14th of February 1861. The proposal arrived after Buchanan left office and was politely declined by Lincoln in his reply dated the 3rd of February 1862.