To be honest, I’m not sure without mirroring your set-up. 
Though you can also default mute the subcategory as an option too (or change the visibility in the security settings). You may need to try a few combinations and see if any get you close to what you want.
If a user Mutes something (a topic, category, tag, or another user) it only counts for them, and has no effect on anyone else.
You’d have to use the default mute options in the Admin setting to make it global, and even then a user could override them.