Who is Hari Singh to decide? He had no authority to rule on the accession when the people revolted. It was the populace that wanted Pakistan to intervene and become a part of it way back in '48 itself. Pak army itself supported those rebels at first, and then joined war later on. And was that wrong? It was what the people wanted. Also read about the standstill agreement state of Jammu and Kashmir had with Pak which precluded the state from entering agreements from other countries. 
What guilt did India do? They supported a ruler who abated the illegal occupation of his people most of whom wanted to side with Pakistan. 
Also, if you're so righteous then do explain what happened with the state of Junagadh, which was a completely opposite case to Kashmir (Muslim ruler; majority Hindu populace). Vallabhbhai Patel wouldn't let the Muslim ruler, Muhammad Mahabat Khanji III become a part of Pakistan even though as you said accession was within his rights. He cut off trade, and eventually when the state was about to collapse, the state had no choice but to enter into an agreement with India. A plebiscite was held and it showed 99.95% of the populace sided with India. Now you do this then flip the script with Kashmir? Talk about double standards and hypocrisy of the highest order.
Kashmir has been a disputed territory to this day. Junagadh is not nor is Balochistan. That's why a plebiscite today would be justice especially considering that there's already been 3 wars due to this land. I guarantee you that the plebiscite would strongly favor Pakistan. But of course the indians don't want that. They quite enjoy occupying land and having their claim to states like Junagadh.
		
		
	 
Now i am surprised you brought  Junagadh & Hyderabad into discussion.Firstly, are they matters of dispute between
India and Pakistan ?Definitely not.isn't it? Both India and PAK might or might not have been 100% legally or morally
correct in the matter of accession of princely states at that time.But are they matter of disputes between IND & PAK like
Kashmir?Definitely not. It is just like a court case  w.r.t a matter between you and me.Both you and me might had done some
other legally wrong things to any other 3rd party.but will the court take those matters into account in the  case of our 
dispute? definitely not. they will consider only the issue between us and trace out as to who is the quilty party in our  
case related to the issue.isn't it? Similarly leave aside the other matters of Junagadh, HYBD etc as they are not matters
of disputes between IND & PAK.And talk about Kashmir only.
Now that you have mentioned Junagadh , i would just point out  another matter of 'Balochistan'.Just google out as to how 
Balochistan was annexed to PAK by Jinnah & co that time.Now w.r.t Junagadh, you yourself has given the answer.whole of 
Junagadh was inside Indian territory.As a result the  Nawab was passively forced to join India. May be a tactical move by
Patel, but no body would blame him  for blocking the trade route with PAK.For instance would PAK allow IND a trade route 
to Afghanistan as a matter of any good gesture( i mean with out any thing in return?) Now i just mentioned these matters
from a moral point of view.As far as these are not at all issues between IND & PAK ,I have no right to be an intense critic 
of the morality of such matters.
Harisingh was the elected ruler of Kashmir by the system that prevailed in those  days just like any nation now a days elects
a government & prime minister. So he naturally  had some authority.Firstly  he used that authority to be an independent 
nation & stay away from  both India & PAK.Secondly when PAK army illegally attacked Kashmir he had no other option but to use 
the same authority to use the rule of 'choosing either India or Pakistan' AND he selected India. Are you so sure that the 
entire populace that time wanted to be with PAK?Yes ,after having experienced  the continuous turmoil since 1948 and 
brainwashing from PAK supported terrotists, now a days majority of Kashmiris might be supporting PAK.But if you state that
was the case in 1948 with a  non so less religious minority population being part of the demography of Kashmir(later removed 
to virtually nil by ethnic cleansing), then sorry man i can't agree with you.PAK govt  & army had no legal right to attack an
independent nation Kashmir that time. It is as simple as that.By the same yard stick do you agree with what Saddam Hussain 
did to Kuwait? Similarly being a PAK citizen yourself  would you support a plebiscite in Balochistan? As i said... i am not
intensely or fiercely arguing with both the above cases as they are not all related to India.Just indicating the moral side 
of these actions as a neutral.