[Nagoyankee]

Wow, you found an answer to your own question at the very end of the post! When we say 食べたくはない, what we mean is "If I could have it MY way, I wouldn't want to eat.". "If I could be honest with my own feeling, I wouldn't want to eat....at least not now. But I will have a piece because it took Mary hours to cook the pie."

What you said in paragraph #2 is not quite correct. It has nothing to do with being formal/informal. You cannot really express either formality or informality with particles. We mostly do it with the verb endings.