Short answers, pulled from the story.
Old Hindi, also known as Khariboli or Dehlavi, was the earliest stage of the Hindustani language. It was spoken by people in the region around Delhi roughly during the 12th-14th centuries, under the Delhi Sultanate.
Old Hindi developed from Shauraseni Prakrit, a middle Indo-Aryan language spoken across part of the Indian subcontinent before the medieval period.
Old Hindi is attested in works by the Indo-Persian Muslim poet Amir Khusrau, the Vaishnava Hindu poet Namdev, and the Sufi Muslim Baba Farid, whose verses appear in the Adi Granth. The Bhakti poet Kabir also used a Khariboli-like dialect.
During the period of Muslim rule in India, Old Hindi acquired loanwords from Persian. That fusion of the Prakritic base with Persian vocabulary produced Hindustani, which later developed into Modern Standard Hindi and Standard Urdu.
Hindi languages were originally written in different variants of Nagari script. Later, the Arabic script in Nastaliq calligraphy was also used.
Old Hindi is a retrospectively coined term used by scholars to label the ancestor of Modern Standard Hindi. The term was not used by speakers themselves; Khariboli and Dehlavi are the historical designations tied to the dialect and its Delhi-centered geography.