Plus, they all come from unions (or at least affairs) between Brits and Indians - the idea that they'd be more racialist than their parents seems off.
They’d be raised in British households, with Indian maids and housekeepers. Of course they’d view themselves as better than the full-blooded Indians, even as a source of pride.
Surely that would depend on the size of the Anglo-Indian population? Especially if they're Christians - it would seem odd in a situation like IOTL (not to kill butterflies) where Muslims and Hindus get their own state, but a larger population of Anglo Indians (say reaching 10 million over 200 years of growth) not receiving the same treatment. (Exactly WHERE that would be is debatable).
Well, first of all, ten million in a country of hundreds of millions isn’t that much. Second, I suspect that this minority would be dispersed across India, making a majority absolutely nowhere. Third, note that Sikhs, another religious minority, didn’t get their own state. This has likely to do with the fact that, before 1947, Sikhs made a majority absolutely nowhere. Anglo-Indians would be similar, except that they would be even more dispersed across India.
I must also note that Partition failed in both of its goals - Pakistan became a racialist state a few decades after independence and split in half due to its acts of genocide caused by racial tension, whereas India continued to, and continues to, have a massive Muslim minority. Even if Britain labelled a region as an Anglo-Indian country, that region would continue to have substantial Hindu and Muslim minorities, and outside it, there would be substantial Anglo-Indian minorities. This isn’t a recipe for stability and India may very well pull a Goa and invade the Anglo-Indian country in the name of the full-blooded Indians.
At the very most, you could see Pondicherry-style or Goa-style regions, where there is substantial European influence and hybrid cultures.
I'm normally cautious about statements about sources that suggest dishonesty (perhaps that is just my reading of your statement). But I'm curious as to what those additional causes were. I can't really try and discuss them without knowing what you're talking about, and who said them.
Causes of the Mutiny include the Doctrine of Lapse, by which the East India Company could proclaim direct rule of any princely state if, essentially, they wanted to, going against Indian traditions of kingship. Other causes include the Indian indenture system, by which Indians were transported for unpaid labour as far as the Caribbean, something which naturally angered Indians, good old hatred of being ruled by foreigners, and rulers wanting to reassert their power. Suffice to say, it’s a lot more complex than the standard historiography of it as “sepoys think their cartridges have pig and cow fat on them, decide to kill all white people”.