Let me go through your questions one at a time.
1. If you want to send a push to all the users that are subscribed to a Channel, I would recommend tagging those users with the Channel name when they subscribe. You can then send a message to that tag informing the users of the new video.
2. Please could you give me some more detail on this use case, I'm not sure if I fully understand what you are trying to do and why?
3. If you were to set these message up as Automated Messages then you would use the Automation API, but I would like to know more about (2) before giving any solid recommendations.