Yes, MP3FileReader emits PCM from its Read method
The chunk size will depend entirely on what the device on the other end of your serial port is expecting.
To convert stereo to mono, you can discard a channel or mix them together. There are StereoToMono classes in NAudio that will help you do this. If you have access to Pluralsight, watch the "Changing Audio Formats" of this course, in which I go into a lot of detail about how to do this.
The chunk size will depend entirely on what the device on the other end of your serial port is expecting.
To convert stereo to mono, you can discard a channel or mix them together. There are StereoToMono classes in NAudio that will help you do this. If you have access to Pluralsight, watch the "Changing Audio Formats" of this course, in which I go into a lot of detail about how to do this.