Exactly as @raintowns said:
No, subscribers don't get notified if you edit an episode, however they do get notified if you add an episode. That's why it's just best to make a new episode with each update (whether it's 1 page or multiple pages in an episode) that way your subscribers know to check it out.
And yep, the "Publish Later" option is my best friend! I have all the pages of my issue 3 comic done, and I have all the pages schedule to release, one at a time, as a new episode every Saturday. That way I don't need to worry about visiting the site every week, I can just keep working on my comic.
How Tapastic names additions to a series as "episodes" was very confusing to me in the beginning too. When I think of episodes, i think of tv and videos, but I guess it makes sense if you think about it in a sense that a new episode can be a single page or multiple pages.
It's all trail and error. Experiment and see what works best for you and your comic.