If your requirement can't be met by sending single characters to control every option then you need to develop a more sophisticated strategy for sending data that will work in every case. As @PaulS suggests, a good idea is to receive all the characters until some end-marker is detected and then analyse what was received to decide what needs to be done. A simple end-marker is a Carriage Return. You can get the Arduino Serial Monitor to add one automatically.
Another good strategy is to separate the function for getting data from the PC from the function that acts on that data. Then both functions will be easier to debug and extend to meet future requirements.
...R