Short answers, pulled from the story.
The specific sound shift began in the 12th century. This process involved raising and lengthening mid vowels when followed by a consonant and a weak yer vowel that eventually disappeared completely.
Tsarist interior minister Pyotr Valuyev issued the decree in 1863 stating there never has been, is not, and never can be a separate Little Russian language. The proclamation banned the use of Ukrainian as a subject and language of instruction in schools across the empire.
The Central Rada of Ukraine issued the Main Rules of Ukrainian orthography on the 17th of January 1918. The Ukrainian Academy of Sciences approved comprehensive rules on the 17th of May 1919 forming the basis for all subsequent revisions.
A variant covering Kharkiv Sumy Luhansk and northern Donetsk alongside Voronezh and Belgorov regions of Russia exists as another dialect mixture. This mixture gradually incorporates more Russian elements moving northward and eastward without clear boundaries.
The Cabinet of Ministers approved an updated orthography on the 22nd of May 2019 bringing back features from 1928. A new version approved on the 14th of November 1989 had previously restored the letter Ge and the accusative case after decades of optional usage.