Muted for 25 minutes, not alone. The video starts as in every session. You switch the microphone off yourself, whenever you like: here silence is the rule, not the exception.
In a silent session there is nothing to say: two people work in the same video call, microphones off, each on their own thing. The video stays on for the whole session, as in every session here; the microphone, on the other hand, you can keep muted from start to finish. If you really don't want the camera on your face, you can point it at your desk instead.
It isn't made for talking: it's made for being there. If you prefer silence, you'll find it's already the norm, not an exception to explain.
The button is there, so yes, for the moment you need it: it is meant for when someone rings the doorbell or walks into the room, not for doing the whole session with the camera off, because a room where nobody can be seen is no longer coworking. The microphone, yes: you can keep it muted from start to finish, and here that's the most normal thing in the world. If you don't want the camera on, the timer and the “Solo” mode open no video call.
No need to say it: in a session each of you works on your own things, in silence or talking, with no explanation needed.
No, it's the same booking as any slot: silence is a choice you make inside the session, not a separate kind of session.