I heard that after the demise of our Prophet (Sallallahu Alaihi wa Sallam), his Namaz-e-Janaza was done by Jibrail, then Mikhail, etc. then other people it seems. Is this true? Please give strong proofs.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

While talking about the funeral prayer on our beloved Prophet Ibn Katheer mentioned in a Hadeeth – which he considered weak – that the angels prayed upon him. Ibn Katheer said, "We already mentioned the Hadeeth reported by Al-Bayhaqi from Al-Ash'ath ibn Tulayq Al-Bazzaar from Al-Asfahaani's Hadeeth, both of them reporting from Murrah from Ibn Mas'ood about the will of the Prophet . He said he should be washed by the men from his family and added: 'Enshroud me in my clothes (these) or in Yamaani ones or in white Egyptian ones.'" He also told them to put him on the edge of his grave and move away from him until the angels pray on him. Then the men of his family should enter and pray on him and then the remaining people should pray one by one (individually). However, there is doubt about the authenticity of this Hadeeth. 

However, there is a consensus that people prayed on him one by one without an Imaam.

As-Suhayli said, "Allaah The Almighty Says in the Quran (what means): {Allaah sends His Salat (graces, honors, blessings, mercy, etc.) on the Prophet (Muhammad) and also His angels too (ask Allaah to bless and forgive him). O you who believe! Send your Salat on (ask Allaah to bless) him (Muhammad), and (you should) greet (salute) him with the Islamic way of greeting (salutation i.e. Assalaamu 'Alaykum).} [Quran 33: 56]. So saying Salat for him after his death is part of this. And the angels are our Imaams in this."

Allaah Knows best.