Just don't see this (particular) film being made. I'm sure Henson bought it mainly to keep it out of other studios' hands. They may eventually embark on a Henson bio but it would be another script (the "Henson" script from a year ago or a whole other one entirely). Despite the attention it's garnered, The Muppet Man would be a disservice to Jim's memory and legacy as it was written by someone who neither really knew him, did the level of research that would earn him anywhere from a "C-" to an "F" if it were graded by an instructor, and simply failed to "get" Jim Henson and what he was about. There's a great movie waiting to be made about Jim but "The Muppet Man" isn't it.
I agree with you totally - no way are they going to use the script "as is" -especially as it has quite a bit of adult/darker stuff. Sounds more like they were interested in doing a Biopic, optioned his script to keep it from being made as a low budget thing and will get people in to re-write it and just give him a title in the credits.
I think for the puppeteers, they might show the actors doing the puppets as well as trying to do the voices. I think the only way it would gel with the real Muppeteers doing the voices is if u never saw the actors actually "underneath" the puppets. We shall see tho - its probably still a long way from being made, if ever.
I think the only way Jim Carey would do it is if he was Jim Henson. Too big a name to be a supporting actor.