I think that pretty much the sole reason, or at least the major reason, that none of the princely states ended up as independent was simply that Delhi knew that should one become independent, the rest would follow. The Indian Princes were pretty damn reactionary most of the time, and at least in India's case I think that it is all or nothing really.
People tend to underestimate the force of Indian nationalism (not a Hindu/Muslim thing, but more of a civilizational-esque kinda thing) and I've noticed especially on this site that people tend to equate India with Europe a lot of the time. While that comparison is not completely untrue as far as diversity goes, as far as the nationalism thing, it is hard to apply because ethnicities and religions are all mixed in together, and creating states for a specific ethnicity or religion doesn't really work well at all and would end up creating a shit ton of conflict.
We Indians are rather prickly when it comes to early 20th century India, I suppose lol.
Yeah, the whole thing was a real tight-rope affair from my reading of the situation and the Indian Princes, while a somewhat mixed bunch, were by and large in the conservative camp - which made the idea of a state under Nehru a rather dismal prospect for them. I think that with India it is an all-or-nothing case as long as we are talking a peaceful transition, but should it turn violent then all the cards are on the table.
Indian nationalism was/is a pretty damn strong force - honestly a bit surprising to me considering the sheer multitudes the country contains - but I think the moment ethno-centrism or forced acculturation comes to the fore (for example if you have Hindu Nationalists trying to cram Hindi down everyone's throats) then it becomes a whole other can of worms.
The thing about ethno-states not being all that great of an idea in regions of mixed ethnicities and religions, and people doing that shit anyway, is sort of a hallmark of ethno-nationalism - isn't it? I mean, just look at Eastern Europe and the absolute shit show which went down in the effort to form consistent ethno-national borders there. I guess having spent a good deal of time in Bangladesh growing up has given me a bit of a different perspective on the power of ethno-nationalism in India - don't think it is a completely crazy proposition if the Dravidians start to pull away if the northerners get too pushy, and there are plenty of linguistic fault lines (thinking particularly the Maratha) which could cause troubles at some point.
Everyone has their bug-bear. Hell, I still have not forgiven Perfidious Albion for the Bombardments of Copenhagen, and I think that is part of what makes us human to some extent.