thank you for your interest in Max2play. I confirm your thoughts about 2, 3 and 4 are correct. With several Max2Play clients and one of them running a Squeezebox Server is a common use case.
Your plans regarding a 5.1 surround sound seem difficult when we think about the signal latency WiFi would cause.
We recommend to acoustically measure the room’s distances with a microphone, find out the best position for your surround sound system, buy a good AV receiver and speakers and use a Raspberry Pi connected to the recevier via HDMI. This way, you can make sure to achieve a good sound quality.
Maximilian from Max2play