I like this idea a lot! Though, I don't think this idea is the best for a documentary. I'm thinking more like a biopic where actors play the Muppeteers and the Muppets wouldn't appear as living beings. I love that Frank Oz talking with Fozzie thing. I think it would work better, actually, if he was doing all this worrying as Fozzie and breaking out the puppet and voice to do the signature "Oh my! What are we gonna do?!"-kinda thing. But I do think there could be a scene where we see the Muppeteers going on set and performing a scene like from the Muppet Show. We would see the scene from the camera viewpoint where we wouldn't see the performers and the backstage viewpoint where we would. And Steve and Eric and all the other real-life Muppeteers would provide the characters' voices while the actors portrayed the Muppeteers... uh... is this a little too confusing. I love the ending, it could stop right after Kermit speaks for the first time in The Muppets Celebrate Jim Henson.
While it should probably heavily focus on Jim and his legacy. It could also feature smaller plots or sub-plots with the other Muppeteers (Frank, Dave, Caroll, Kevin) getting picked and discovered. We could also see a little bit to their backstories too. We already know Kevin's now that Being Elmo is out.
As for the title, maybe it should be called Behind the Frog or Fleece and Foam. I don't know, this is just my... wow... my really long take on this! Great idea!