Always like hearing about other writers'/creators' process. I've honestly found that having multiple projects helps, rather than hurts productivity. Sounds like you have a very cool creative back-and-forth collaboration going.
Funny your comment on the beginnings being flexible. For a project I'm working on currently, I've already pushed the beginning backwards in time twice from my original start point.
And I hear you on the teacher thing...as a former elementary teacher, there never seems to be enough time in the day and there's always something you could be doing to better yourself in the classroom. But, striking a personal balance is a key to being effective as well, and if you're someone who needs to create to be happy, you need to make time for it.