Here are the transformations based on your provided sentences:

1. We really wish you had kept this plan a secret. (GIVEN)
→ We would sooner have had this plan kept a secret.

2. When the book was published, the writer was greatly criticised. (CAME)
→ Upon the publication of the book, the writer came in for great criticism.

3. It is possible that John visited Mary two days ago. (DROPPED)
→ John may have dropped by to visit Mary two days ago.

4. More and more people go on a diet now.
→ There are more and more people going on a diet now.

5. What a pity you missed the party last weekend.
→ I would prefer you had attended the party last weekend.

6. They redecorated the living room, which was unnecessary. (NEEDN'T)
→ They needn't have redecorated the living room.

7. I am sure they will announce the result soon. (MADE)
→ There is every reason to believe that they will make the announcement about the result soon.

8. Her friendliness made her the most popular girl in the class.
→ But for her friendliness, she wouldn't have been the most popular girl in the class.

9. It was because of her friendliness that she was the most popular girl in the class.
→ It was because of her friendliness that she was the most popular girl in the class. (You had indicated this phrase already, so it remains unchanged.)

10. She doesn't have to do this exercise. (NEEDN'T)
→ She needn't do this exercise.

Let me know if you need further assistance!