Great ideas for his longivity you've got there.
Will you be using Antron fleece for your Kermit's skin? I'm pretty sure the vintage Fisher Price Kermit's were made using actual Antron fleece, and I've washed mine a few times without problems.
Also, if you don't mind saying, what details are you waiting to find out about Kermit? It's obvious you want a very perfect Kermit, and that is so cool! When I finally get around to making mine, you can be sure I'll be referencing this thread.
Cheers!