How does Romeo’s overhearing and then responding to Juliet’s private confession of love (she does not know that he is there, hence listening to her) affect the development of their relationship? That is, how would things progress if Romeo had never heard Juliet say she loves him?

Answer 1

https://www.sparknotes.com/shakespeare/romeojuliet/
You might check out the different character analyses here.

The rest is up to you. No one can write this assignment for you since it's a "what if" type of topic. Only you can apply your understanding of the play to answering your second question.