Many employees of the English east India company had to spend the majority of their time on the subcontinent due to the enormous time taken to sail back to the British isles. With these officers being "stuck" in India, how did they adapt to Indian life and assimilate to the local culture?
They didn't. Most people of this social class would have not dealt frequently with actual Indian people, aside from their servants. These people would have mostly socialized with other white, British transplants. Indian food did become quite popular, which is part of the reason curries are so popular in England today.
Until the 1830's they often assimilated completely in their private lives. Certainly up to the early 1820s marrying an Indian lady was acceptable, indeed, some became so Indianised that they adopted the zenana. Generals like Sir David Ochterlony became, effectively, Moghuls, (having 13 wives and living entirely as a Moghul at home) whilst still carrying out their duties for the HEIC. Sir James Skinner, himself the son of a British officer and a Rajput lady, served in the HEIC and had 14 wives, some Indian and some European, his sisters all married HEIC officers, and a descendant of his, Michael Skinner (an Indian) currently (or until recently) commanded Skinner's Horse in the Indian Army. Certainly in the 18th century your premise is correct. Very few British or European women went to India, so most, if not all, HEIC officers married Indians, or people of mixed heritage, usually at the time referred to as "Portuguese". Once British women started coming to India in numbers, from the 1820's onwards, the whole situation changed and British officers became far more racially aware and racially exclusive. Philip Mason's "A Matter of Honour", Richard Holmes' "Sahibs" and William Dalrymple's "White Mughals" are all very good on this. Interestingly, John Masters, who wrote extensively about British India, having served there in the Ghurkas, and who wrote sympathetically, and almost uniquely in that respect, about the Anglo-Indians, discovered in much later life that he had an Indian ancestor. Lord Liverpool, prime minister from 1812 to 1827, had Indian ancestry, a grandmother being described as "Portuguese", meaning a Indian christian of mixed race.
They didn't; there were quarters and bungalows built specially for the English that were designed to isolate them and not let any "native" come in contact with them.
I forget which road it was, but there was a popular seaside road(a promenade of sorts) which was off limits to Indians after 4 in the evening because the Brits would come there for their evening walks along with their women.