Short answers, pulled from the story.
The first known description of the six-syllable mantra appears in the Kāraņavyūha Sūtra, a text dating from the 4th to 5th centuries. This Mahayana scripture presents the chant as the innermost heart of Avalokiteshvara, the Bodhisattva of compassion.
Scholars have debated the grammatical parsing of the middle section for over a century with Donald Lopez citing Tibetan grammatical sources suggesting the word mani padme functions as a noun of address in the vocative case. Damien Keown notes evidence from texts depicting the mantra itself as a female deity with autumn yellow skin and four arms.
The mantra was first introduced into Chinese Buddhism during the Song dynasty between 960 and 1279 CE. In 983 CE, the monk Tianxizai translated the Kāraņavyūha Sūtra into Chinese bringing the chant to East Asian shores.
The text describes the mantra as the condensed form of all Buddhist teachings and calls it the sadaksara or six syllabled. The six syllables appear across dozens of scripts throughout Asia with varying phonetic renderings including Sanskrit writing which lacks capital letters so transliterated versions range from all caps to initial caps or no caps depending on the source.
The largest mantra inscription in the world sits on Dogee Mountain in Kyzyl Russia. A stele erected at the Mogao Caves in 1348 commemorates donations from Sulaiman Prince of Xining and includes the six-syllable mantra written in six different scripts including Lantsa Tibetan Uighur Phags-pa Tangut and Chinese.