1- Yeah seems be agreeing with the statement as a whole- just agreeing with the speaker to push the conversation forward, and not answering a question.
2- I think you have got the wrong part of the sentence- the person is confirming that they are to be married without telling them:
Yes, I am to be married without telling you.
Also, remember that a lot of speech doesn't follow the strict rules of grammar, so you will hear things being said that contradict the rule you read.
Student or Learner