They will if you have a visa. It's very simple. As a passenger, you are responsible for your visa business. You must be prepared to show your visa to airline staff upon check-in without them asking you. Look at the staff and if they miss your visa, show them.
Nothing wrong with being asked how long you will be staying for, particularly for visa-exempt folk.
US immigration asks this question (or similar ones) of most arrivals and it's the same in the case of EU/Schengen, UK, Australian and New Zealand immigration.
No, of course not. You'll either always be interviewed, based on your past history and have to put up with it or be even more likely to be interviewed should you spend as little time as possible outside of the country.