LOL.
As usual, you have a formidable argument. If you had only been on the Indian side!
Your argument is flawed in one minute but all-important respect: the definition of terrus nullius. It does not mean a territory shifting its loyalty and allegiance from one ruler to another, nor does it mean a territory with an administration that can be browbeaten and overthrown. It means a territory where is no administration, no state apparatus, and no recognised or capable of being recognised sovereign territory.
To be precise, the most famous example is Australia; another very good example was Antarctica.
Gilgit (I shall not talk about Baltistan; for some peculiar reason, both Pakistanis and Indians hyphenate Gilgit-Baltistan, where there is nothing about their situation that invites hyphenation) was not without administration, or a system of governance. It was governed by the British directly, under 'lease' from the Maharaja, until the lease was surrendered on the eve of independence, as you have meticulously pointed out. It slipped out of the Maharaja's grip, again, as you have pointed out; it did not slip into the grip of Pakistan, and its position remained independent at the time.
If the Maharaja had overrun it militarily, the question would have remained moot; he didn't, so his people had effectively seceded.
A seceded state is emphatically not a terrus nullius. An uninhabited, or unadministered state is.
Pakistan's rights over these Emirates hinges on the definition of the states offered a choice between Pakistan and India. Were only those states in direct treaty arrangement with the Crown to be given a choice? If so, then Hunza and Nagar were to travel along with their masters, the Maharaja of J&K. Were the subordinate states also to be given that choice? Both legally and logically, the answer to this would be 'NO'.
Why?
Because the choice was explicitly given only those who were in direct treaty with the Crown.
So Hunza and Nagar technically had no freedom to choose other than what was done for the entire J&K State.
I would like to leave you with this bonbon.