I hope you're not basing your opinion on much later artistic representations?? That would be insane. They are not photos.......
But you do have a point about Guru Arjan Devi ji's shaheedi. Jahangir (in his memoirs) specifically mentions that he ordered Guru ji to be executed in line with some ancient (from Genghiz Khan's time) little known Mongol laws called 'yasha', and as far as I can tell, under these protocols nobles were to be executed without having their blood spill on the ground.
I have read of Singhs being hanged from trees by their kesh in 18th century Persian accounts of conflicts with Singhs. If the whole aim is to terrify and destroy your captives, I doubt the persecutors would have cared too much about keeping their prisoners dastaars on - if anything I would imagine they would have gone out of their way to remove them, to try and degrade them further.
Does anyone know from historical written sources, if it is specified whether Singhs were allowed to keep their head covered when persecuted by the Mughals, during torture/execution?
Was it just on a case by case basis?
For Guru Arjan Dev Ji, it seems head was allowed to be kept covered with Dastaar.
most of the photos I’ve seen Banda Singh Bahadur with head uncovered