Short answers, pulled from the story.
The Kingdom of Rohilkhand was founded by Ali Mohammed Khan in 1721. Ali Mohammed was a Jat boy adopted at age eight by Sardar Daud Khan Rohilla of the Barech tribe. He took command of his foster father's militia at fourteen and carved the kingdom from the declining Mughal Empire.
Ali Mohammed Khan was Jat by birth, not Afghan. He was adopted by an Afghan tribal chief and later gained recognition as a Rohilla chief through his military and political achievements. His descendants in Rampur later claimed he was a Barha Sayyid, but could produce no valid pedigree to support the claim.
Rohilkhand fell because the Rohillas could not repay Shuja-ud-Daula, Nawab of Oudh, for military help against the Maratha Empire in 1772. Shuja-ud-Daula then allied with the British and paid Warren Hastings forty lakhs of rupees for military support. Colonel Alexander Champion defeated the Rohilla forces at Miranpur Katra on the 23rd of April 1774, killing their leader Hafiz Rahmat Khan.
Before his death in 1749, Ali Mohammed Khan made Hafiz Rehmat Khan swear on the Koran to act as guardian of Rohilkhand until Ali Mohammed's sons came of age. The oath was meant to hold until those sons reached adulthood. By 1754, Hafiz Rehmat broke it and appropriated the most valuable portions of the kingdom for himself.
Faizullah Khan, the eldest surviving son of Ali Mohammed Khan, negotiated a separate treaty and retained his territory around Rampur. On the 7th of October 1774, the Rohilla State of Rampur was formally established in the presence of British Commander Colonel Champion. The same Rohilla dynasty continued to rule Rampur as a pliant state under British protection.
About a quarter of Rohilkhand's population were Muslim converts, while the majority of inhabitants were Hindu. The Rohilla rulers were Muslim, but they governed a predominantly non-Muslim state, an unusual feature for a kingdom built on Afghan Muslim immigration.