for number 2, the messenger services needs to be set to running on their machine. It is turned off by default.
For number 1, I believe you need to get in their machine via admin share first(So it knows you have access to their machine.. Dont want any old person shutting down PC's) or if its on a domain, be an administrator. Failing that, look for PSShutdown, hell might aswell get PStools while you're at it.