Like all Sampardas, modern Nihangs claim a lineage directly to Guru Gobind Singh Sahib via Baba Binod Singh, who was appointed by Guru Sahib as Jathedar of the Panj Piare under Baba Banda Singh. The next 3 Jathedars in this lineage are just the leaders of the Dal Khalsa, the combined army of the Khalsa. Then the real lineage of the Nihangs starts with Akali Baba Naina Singh. The truth is, there is no actual link between Sardar Jassa Singh Ahluvalia and Akali Baba Naina Singh, the supposed 4th and 5th Jathedars of the Buddha Dal. This connection is a fiction created to link modern Nihangs to early Panthik leadership.
Sardar Jassa Singh Ahluvalia was the head of the combined forces of the Khalsa, the Dal Khalsa. He was also the Misldar of the Ahluvalia Misl. After his death his cousin Bhag Singh became the next head of the Misl. Bhag Singh’s son, Fateh Singh, became the third head of the Misl in 1801. Jassa Singh didn't appoint anyone as leader of the Dal Khalsa before his death and his Misl continued into the colonial era.
Baba Naina Singh, on the other hand, was not the head of any Misl. He was simply a respected soldier in the Shahid Misl, which developed out of Baba Dip Singh's old Jatha. After Baba Dip Singh, Shud Singh became the head of the Shahid Misl. After his Shahidi, Karam Singh became the Misldar. In 1795, Gulab Singh became the head of the Shahid Misl. Naina Singh was not the head of Jassa Singh’s Ahluvalia Misl, nor was he the head of the Shahid Misl, of which he was simply a member.
We actually know Baba Naina Singh's history. He was first based out of Damdama Sahib when he was part of the Shahid Misl. Then he moved to Anandpur Sahib, where he started his own Jatha and where he raised Phula Singh. Later, Akali Phula Singh moved the Jatha to Amritsar. Baba Naina Singh's samadh is on the parkarma of Bibeksar.
There is no historic source that links Jassa Singh to Naina Singh. None at all. Not one. They may not have ever even met.