Can you clarify what you mean by India being united since the Mauryas? I am no expert on ancient Indian history, but this doesn't seem to be true in general, and it certainly isn't true of the period immediately preceding the EIC conquests.

This is part of the answer to the second half of the question - the EIC made strategic use of the existing divisions among Indian polities. The Kingdom of Mysore and the Maratha Empire were not united with the Mughal Empire, the Nawabs of Bengal became briefly independent, as did the Sikhs, and so on. These polities regularly went to war with each other and sometimes made European alliances or hired European mercenaries in the process. These were not citizens of a single democratic polity, but subjects of various Empires. This was well prior to any widespread "Indian" nationalism that made the various groups think of themselves as "countrymen." (Indeed, it is prior to the quasi-legendary national origin stories of most European nations, excepting perhaps the English and Dutch.) There is little reason to suppose the rank and file had any greater concern about fighting for Europeans than anyone else, provided they paid their salaries on time, and Europeans fought for Indian rulers on much the same terms.