1.) You can't, anything that is specifically new, as in not an edit, will have your readers notified.
2.) As I said, any time you post a new episode, your readers get a notification. Now if you post in batches, your readers will always get the most recent episode update. This means that if you post 5 episodes, then your readers will get one update, and that update is whatever one it is currently on. Maybe they'll sign on and see Episode 04 update which will then eventually be replace by Episode 05 next time.
3.) You can't add episodes before other episodes. Apparently in Tapas' great wisdom, they thought that being able to move your episodes around was an inconvenience. Sorry Tapas, but this is something Smackjeeves and ComicFury, two far lesser sites can do. This is something you people at the site really got to work on.
4.) Its up to you. If you want to add it to the end of an episode, go right a head. If you want a completely new episode for it, just post it there. I'll sometimes schedule any of my news about a minute after my episode post, that way readers still have material to read.