I thought you were going in a different direction with this. To me, sensitive questions are very much in the first observation you pointed out (known unknowns). i.e. things we know we don't know about somebody. These things are often sensitive and therefore avoided in casual conversation. Not every known unknown is a sensitive questions, but a question by definition implies an unknowing. You ask so you can discover (unless you're perhaps asking as a conversation starter in a particular topic).

I'm not sure I follow what you mean by "unknown knows". How is a sensitive question an unknown known?