Sorry, invalid according to constitutional law.
The India Independence Act did not stipulate any additional procedures. Any Tom, Dickie or Harry could have suggested additional procedures - it's a free country, after all - and Dickie Mountbatten did. That in no way makes the Maharaja's sovereign right invalid in any way, since he adhered to the conditions that the British Crown had conveyed through its representative, the Viceroy: either of the two Dominions, no question of any more, meaning, no independence for any princely state, including Hyderabad, and a choice between India and Pakistan only; and a choice to be exercised with contiguity in mind.
Hari Singh handed over three, and only three rights to the Union of India. Those belong irrevocably to the Union of India. He retained the rest. These he devolved on the Assembly of the State of J&K that was formed on the basis of the J&K State Constitution. So the sovereignty lies divided between the Union of India and the State Legislative Assembly of J&K State, and the plebiscite proposed by Mountbatten and confirmed by Nehru, and taken up by the UN is not a mandatory part of the procedure of devolution of the sovereign power but only a proposal, a recommendation that was rendered infructuous by the resistance off the battlefield of an aggressor power.
In Junagadh, the Nawab acceded irregularly; there was no contiguity, so he could not have acceded to Pakistan, any more than could have Bhopal. His sovereignty did not vest in the people of his state because he gave it to them. They took it from him by vote in the plebiscite.
In Hyderabad, the Nizam held out against accession in irregular fashion. Independence was not an option, but he and the Razakars thought they could make it one.They too found out the hard way the difference between rioting and military action.