I do not know about the incident mentioned above (it could be true because Raina may have been IG's favourite with him also being a kashmiri) but Shahbeg Singh reportedly exhibited separatist tendencies as early as 1971 during the war. Gen. Niazi mentioned this in his autobigraphy.
He said that Shahbeg Singh came to him after the surrender in Calcutta? with a map of undivided Punjab and said that Sikhs had lost out big time during partition and was quite upset about it.
General Shabeg Singh was direct descendant of Bhai Mehtab Singh. Bhai Mehtab Singh was a Sikh hero who entered Golden Temple when it was occupied by Mughals and killed the Mughal officer.
As well as divided punjab is concerned all sikhs are upset with Partition since it left out Nankana Sahib, Panja Sahib, Shaheedganj as well as
lots of places associated with Ranjit Singh, Hari Singh, Akali Phula Singh,etc. So does that mean that all these sikhs are "separatists". and Gen. Niazi who meekly surrendered is not the person to judge Shabeg Singh who faced the mighty Indian army without surrendering.
Here are some basic facts about Shabeg Singh.
1. General Shabeg Singh cut off his hair in 1971 and trained Mukti Bahini which was highly successful. (This act alone prooves his commitment to India) He willingly cut off his hair so that he could move around with mukti bahini guerillas while training them.
2. General Shabeg Singh was decorated for his efforts in 1971 war.
3. General Shabeg Singh ordered investigations into the money spent for the marriage.
4. General Shabeg Singh was cashiered out of army one day before his retirement because he accepted a RS 2500 worth gift item
from his junior officers.
5. General Shabeg Singh lived in Chandigarh and at around 1978-79 he became active in Akali politics.
6. General Shabeg Singh was fed up with Akalis and got himself aligned with Jarnail singh Bhindrenwale at Golden Temple.
7. Nobody in their wildest dreamed believed that Indian army will entered the Golden Temple but they still took precautions by fortifying it.
(Golden Temple has historically been fortified).
8. Indian army entered the Golden Temple on the martyrdom day of the Fifth Guru who created Golden Temple. This act itself enraged the Sikhs all over the world. Imagine Mahmood Ghaznavi attacking Somnath temple on Janamashtami? or attacking Ayodhya on Ramnavmi?
9. Golden Temple is created on the lowered ground so as to depict humility as oppose to Hindu/Muslim/Christian temples that are created on a platform thus it was much easier for defenders to fortify it.
10. For majority of sikhs General Shabeg Singh attained martyrdom at Golden Temple in the similar fashion as his ancestor Bhai Mehtab Singh and hundredsd of thousands others.
In my eyes General Shabeg Singh is a perfect soldier since he lived and died as a soldier with firm commitment to his ideals.