My question involves employment and labor law for the state of: Delaware

Hello! I work in the medical field as an office manager. I oversee two practices with a total of 6 employees. I am not the "big" decision maker; if someone came in and asked for the "boss", then that person would be directed to the doctor. I am the liaison between the staff and the doctor and I supervise the employees and the day to day operations of the office. In addition, due to my knowledge and experience in my field, I have been given a few more responsibilities and was told that they would only be temporary until someone was hired to take them over and that was told to me 2 years ago. That being said, I am the acting physical therapist, the financial officer, I initiate doctor's notes, and I do the intake and history with new patients (PT should be a separate job and the notes and intake were supposed to be taken over by the doctor). I have been spread thin. We recently changed our clinic hours, which makes me work a minimum of 42 hours per week. I work on average 45 hours per week; sometimes a lot more if I am doing outside marketing events or taking work home with me. My employer just came to me and told me that he was changing me from hourly to salary and basing that on my hourly wage x 40 hrs/week x 50 weeks. Changing me to salary just gave me a $5,000+ a year pay cut.

My questions are:

1. Can he use that formula to base my salary if our clinic's office hours are open to see patients for 42 hrs/week?
2. Because I am performing the job duties of what should be multiple employees, am I entitled to overtime or be paid more than one salary?

Thank you for taking the time to help.