DItto on the schedules! If you want to check out my advice on managing your day, check out my blog here: http://lifeinjapan.blogdns.com/?page_id=869
Scroll to the bottom to see the organizational charts I have made that could help you out. I presented these during Mid-Year Conference last year and people said they found them useful.
Once you get officially notified, I would suggest planning a meeting with ALL of your JTEs to discuss what their goals will be with having you in their classroom as you will only be there once in a while now. State that it will be impossible for you to plan 100% of the lessons if you have to do specific things for each class. It will be really easy for you to plan 100% of the lessons if all the students do the same thing, but just tinker it for different ability levels. Of course you will have to do different lessons for ES and JHS, but if you think about it, that might mean planning 2 lessons (1 for ES and 1 for JHS) per every 2 weeks or so, depending on your rotation. Many ALTs I know who are on heavy rotation (haha) end up with 4-6 classes a day because every school wants to get the most out of you. They usually let them do the same thing with each class because of this.
I am in the same boat as you where I am the main teacher essentially, and the JTE is like my ALT. This is not a usual circumstance, but many people would kill for that instead of being a typical assistant. It will look much better on a resume for sure. Once you start to get overwhelmed, I find it helps to spend any spare time hoarding lesson plans from online sources. Going on trips during times with no classes also helped me feel less stressed.
On a side note, I heard teaching ES totally rocks. I was hoping to get to visit an ES once a week or so, but I am only a HS teacher. I have to watch my friends have all the fun...