I love your posts, but I think you may be off in this line of thinking.
15 million people were butchered during the Partition. I don't know what scale you're thinking of, but it was unnecessarily bloody 100%.

I think you're conflating the number of deaths with the number of people displaced. Displaced persons were between 12-20 million. Deaths were at between several hundred thousand or up to two million.

en.wikipedia.org - Partition of India

I think both of those numbers are vastly lower than what would have occurred if India had not been partitions, and the British and Indian representatives (Nehru, Patel, Gandhi) had tried to keep the Muslim areas locked into the new country. There would have inevitably been civil war (violence had already started even in anticipation of the Muslim areas not being given their own country. It would have been horrific.

And to your other point, almost all successful empires have always been multiethnic. The Roman empire, the Persian empires(#1 and #2), the British empire, you name it.

They were no less capable of horrors and brutality than a nation state, and typically reserved governance to the home country (typically ruled by a discrete and coherent people distinct from the ruled). The Romans weren't immune to genocide or ethnic cleansing (Carthage, Judaea, the Epirus) - nor were the Persians or Mongols or the Greeks or Ottomans (sorry, Armenia).