Your reasoning for the force spirits sounds good to me. I like the "ultimate secret" aspect of it. Perhaps having Anakin appear at some point could be explored, but other than that it seems you've nailed the concept. I would imagine Yoda would inform Obi-Wan of the immortality technique before he goes off to Tatooine though, as once he takes Luke there he's there for good until ANH.

Also, I see you have decided on the trilogy-of-trilogies structure. An excellent choice.

I also see you've had some thoughts on the timespan of the saga. When I was plotting out the trilogies I had a dilemma with the time in between episodes 6 and 7. I'd decided on a two-year gap, granting my Episode 6 was ESB, as I didn't want Han to stay frozen in Carbonite for too long a time. I don't know what your thoughts on this are exactly, but don't you think another 20 year gap is a bit long? I mean, between 3 and 4 it makes sense due to the Jedi having to go into hiding, waiting for the twins to come of age, but what would happen in that second 20 year hiatus?

CRAZY IDEA: As I was typing this up, the thought occured to me. There are many stories about people being frozen and revived in their future. What if this scenario played out for Solo, being dethawed 20 years in his future? This would be a great way to bring the audience up to speed on the events of the last two decades, using Han as the excuse for expositional dialogue explaining what's happened since episode 6. Also, this would serve as a way to have Leia grow out of Han and into Luke. Abstract ideas I know, but it's a start. What do you think?