> Using an example from the doc, if your child process accesses "/dev/class/framebuffer", can you intercept its communications?

Yes. When creating the namespace for the child, the parent can map names to what whatever communication channels it chooses. If the parent wants to interpose on the child's access to "/dev/class/framebuffer", the parent could map that name to a channel that leads back to the parent.

> Can a process create a custom sandbox and run, say, AppMgr with limited permission to limit the permissions of all apps it manages?

Yes. That's useful for testing as well as for sandboxing.