Hi Raj,

Its nice that you raised this issue... :)

History indeed can be distorted and manipulated to suit the people with vested interests... Anybody, who is ruling the roost twists the history to suit their purpose. Even today we find people like indian ex-education minister, murli manohar joshi, changing sikh history books to demean the contribution of Sikh Gurus. So, its not a big deal if history is distorted. Sikh are open for debate and if some facts are established, then these facts are perfectly acceptable to us. We do not stick to fabrications inducted in Sikh history over the centuries. If the facts are established then thats perfectly alright for us.

But one that can never be manipulated is Granth Sahib Ji... the perpetual Sikh Guru. Sikh Gurus made it absolutely sure that nobody could manipulate Granth Sahib. Even then we have some contraversy over whether Raagmala is part of Gurbani or not. Although we have clear verdict over the issue.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/history-of-sikhism/200-harimandir-sahib.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=200

But, we do not have to defend anything written 100s year after the demise of these people like Jesus & Muhammad (pbuh) and believe the scripture like Bible or Quran represent the true picture of these persons credentials. Witnessess can be easliy manipulated... Eveything can be manipulated but Gurubani can never be manipulated...

Anyways the issue that you raise is up for discussion.

Best Regards