They wont cast a Muppeteer. They will cast an actor who they beleive can convey the story set out in the screen play.
They have a very small budget, so most of your suggestions are not valid, especially Carey.
They have optioned the screenplay, and thats about as far as itgoes. Screen plays get optioned all the time, and never see the light of day... so I wouldnt go getting your knickers in a twist over this one.... and when and if it does happen, you can be sure of one thing.... if Di$ney dont want to play ball, it will pretty much fall flat on its face without the Muppets..
Then, you wil need to find actors who will look as if they can puppeteer in camera.... unless of course you dont show a puppeteered performance AND the actor playing the puppeteer in the same shot. That narrows your choices down a lot as you will need to find an actor who can A: Act, B: Puppeteer and C: do a passable Muppet performance.
B&C are pretty crucial, and for thatto happen you will need to coach them. That means finnding a puppeteer who is willing to teach the unteachable (you either can do it or you cant). Then, for any set peices that require Puppet staging and choreography, you will need to find someone that can now wrangle a Muppet performance from these "newcomers" to a standard of performance that mimics the masters. Now, I have seen first hand how Kevin Clash can polish an existing puppeteers performance at a Muppet Workshop. Heck, I went in as an experienced puppeteer and came out two weeks later feeling like I had just learnt the meaning of life itself! But the flip side is that I have also seen what happens when you try and teach someone who just doesnt "get it". As the saying goes, you cant polish a turd... so casting is crucial, because you have to get not only the look and sound as close as you can, but also the skillset as well... Unlike a Marvin Gaye Biopic, Bobbay Darrin or the doors, you cant fudge something as obvious as a manually based performance. Look at Geoffry Rush in SHINE. He actually learnt how to "mime" the piano peices so the right fingers hit the right notes....
Mmmm, Geoffry Rush.... Now there is an intelligent casting call that could work (were it me making said call). (but im not).
Add to that suggestion Bill Nighy. Needs someone capable of a performance. Think Kurt Russell in Elvis or Gary Busey in the Buddy Holly story.... It needs to be that good.
So, personally speaking, I would have to look at and address all of those technical issues before anything else.... They obviously have a script, they are obviously "talking" about this, with statements of intent in the press release..... but if they dont approach this carefully it will blow.... If you could do a Jim Henson Biopoic WITHOUT the use of Puppets, Muppets and creatures, it would be a walk in the park..... but unfortunatly they have to feature those aspects in some shape, one way or another and with that come many hurdles, both legally and technically.
I just hope that whoever they hire as Puppet Performance co-ordinator is a perfectionist and manages to craft some unique performances from whoever he / she has to coordinate.