1) Do you say "We can stay here until late night" or "We can stay here until late at night/late in the night" or "We can stay here late into the night"? You might say "We can stay here very late tonight." 2) What about "we will talk about this later at night/in the night"? And "I won't leave until early afteenoon/early in the afternoon"?"? We will talk about this later tonight. I won't leave until the early afternoon. 3) I know you say "the boss's fair-haired boy." and "The lowest man on the tottem pole" ... But what if the person is a woman? "The boss's fair haired girl"? "The lowest woman on the tottem pole"? Either. People would understand if you used boy/man but you could change it too.

Use AT NIGHT when talking about nights in general: "I sleep at night." IN THE NIGHT is used when talking about a longer period of time, and it can be substituted with DURING THE NIGHT. 1) We can stay here until late at night. 2) We will talk about this later in the night. (=later during the night) I won't leave until early afternoon. 3) Those are idiomatic expressions, so leave them be.