Sorry for not being clear in my previous post I did not explain significance of the sources
I cited as I wrote in a rush.
In Bansvalinama, there is mention that some Singhs approached Guru Sahib after Shaheed Bhai Mani Singh Jis
shaheedi. They told GUru Sahib, if it pleases you then we will make one granth meaning they would include the writings of Guru Gobind Gobind SIngh Jee to Sri Guru Granth Sahib JI .Guru Sahib replied,
ਆਦਿ ਗੁਰੂ ਹੈ ਗ੍ਰੰਥ । ਇਹ ਅਸਾਡੀ ਹੈ ਖੇਡ, ਜੁਦਾ ਰਹੇ ਮਨ ਮੰਥ Sri Guru Granth Sahib Ji is Aad Guru, while Sri Dasam Granth Ji is Gurus " Khel" ( play) and both should be kept seperate. . This did not mean there was no respect to the pothis of Sri Dasam Granth it simply meant one cannot change Aad Sri Guru Granth Sahib Ji which was compiled by Aaad Guru ( Guru Parmeshar)
Sri Dasam Granth Sahib Ji was originally complied by Shaheed Bhai Mani Singh Ji. The Singhs did not know what to do with this compliation; whether they should keep it as one granth or keep it as seperate volumes as Guru Sahib wrote them. Bhai Mehtab Singh Ji and Bhai Sukha SIngh Ji decided if they returned back with dushtman Massa Ranghars head then they would take it as a good omen to keep Sri Dasam Granth as a single volume.
How do we know Guru Sahib actually ever approved this granth. Guru Sahib gave darshan to Bhai Desa Singh Ji ( shaheed Bhai Mani Singh Jis grandson) in his dream while Bhai Desa Ji was resting from a yatra to Patna Sahib ( birth place of 10thking) . Guru Sahib told Bhai Desa Singh Ji the exact order of banis in Sri Dasam Granth according to order he recited them.
ਸੁਨਹੁ ਸਿੰਘ ਇਕ ਬਚਨ ਹਮਾਰਾ। ਪ੍ਰਥਮੇ ਹਮ ਨੇ ਜਾਪੁ ਉਚਾਰਾ।
ਪੁਨ ਅਕਾਲ ਉਸਤਤਿ ਜੋ ਕਹੀ। ਬੇਦ ਸਮਾਨ ਪਾਠ ਜੋ ਅਹੀ।
ਪੁਨ ਬਚਿਤ੍ਰ ਨਾਟਕ ਬਨਵਾਯੋ। ਸੋਢਿ ਬੰਧ ਜਹ ਕਤਾ ਸੁਹਾਯੋ।
ਪੁਨ ਕੋ ਚੰਡੀ ਚਰਿਤ੍ਰ ਬਣਾਏ। ਅਮਤਰ ਕੇ ਸਭ ਕਬਿ ਮਨ ਭਏ।
ਗਯਾਨ ਪ੍ਰਬੋਧ ਹਮ ਕਹਾ। ਜਸ ਪਾਠ ਕਰ ਹਰਿ ਪਦ ਲਹਾ।
ਪੁਨ ਚੌਬੀਸ ਅਵਤਾਰ ਕਹਾਨੀ। ਬਰਨਨ ਕਰਾ ਸਮਝੀ ਸਭ ਗਯਾਨੰ।
ਦੱਤਾਤ੍ਰੇਯਾ ਕੇ ਗੁਰੁ ਸੁਨਾਏ। ਪੁਨ ਬਚਿਤਰ ਬਖਯਾਨ ਬਨਾਏ।
ਤ੍ਰਿਨ ਕੋ ਭੀ ਇਕ ਗ੍ਰੰਥ ਬਖਾਨਾ। ਪੜ੍ਹੇ ਮੂੜ੍ਹ ਸੇ ਹੋਇ ਸਯਾਨਾ।
ਸ਼ਬਦ ਹਜ਼ਾਰੇ ਕੇ ਸੁਖਦਾਈ। ਸਬੈ ਨ੍ਰਿਪਨ ਕੀ ਕਥਾ ਸੁਨਾਈ।ਜੋ ਮੈਂ ਹਿਤ ਕਰਿ ਬਰਨ ਸਵਾਰੀ। ਪੁਨ ਕਹ ਗਤ ਨ ਕਹੂੰ ਉਚਾਰੀ।
ਚਾਰ ਸੈ ਚਾਰ ਚਰਿਤ੍ਰ ਬਨਾਏ ਜਹਾਂ ਜੁਣਤਿਨ ਕੇ ਛਲ ਦਿਖਰਾਏ।
Bhai Desa Singh Ji rehatnama
. in addition he gave the following Hukum in which one should select and memroize banis from " both granths"
ਦੁਹੂ ਗ੍ਰੰਥ ਮੈਂ ਬਾਣੀ ਜੋਈ॥ ਚੁੰਨ ਚੁੰਨ ਕੰਠ ਕਰੇ ਨਿਤ ਸੋਈ॥
Bhai Desa Singh Ji rehatnama
So it is evident that Sri Dasam Granth is a granth that has been accepeted by Guru Sahib and the panth. Out of fear, the British challanged its authneticty with the help of their " loyal Sikhs"