Why didn't Hinode turn fully into a cat when a cat mask is placed on him? As Muge's failed transformation back into a human has shown, the transformation works depending on how dissatisfied the human is with their life. As shown in the film, Hinode isn't dissatisfied with his human life enough compared to Muge for it to work fully, also only his hands were the ones being transformed which fits with the theme of his insecurities on being able to continue doing pottery.
This adds another layer of Fridge Brilliance, one that borders on Fridge Horror. The mask can transform you successfully based on how much you feel your loved ones don't love you. Hinode's hands becoming feline represents he feels his family won't accept him for his love of pottery. Compared to Hinode (whose at least has an entire loving family), Muge wholly transforming into a cat sadly represents she feels her family (specifically her biological mother) doesn't love any part of her at all. If anything, this is a sobering testament of how badly her mother's leaving damaged Muge's self esteem.
Muge's stunt as Taro the cat is effectively a literal version of catfishing. This was never called out on.
As Muge remained trapped in her alter ego as Taro the cat without her human face, she begins to lose her understanding of human speech. If she had been stuck longer as such, how much more of her inner humanity would have been lost? Was that why there is a bar operated by the Mask Seller's previous customers/victims at the cat island? To preserve their inner humanity and stave off becoming dumb animals?
According to Kinako, she wasn't the first cat to make the deal with the Mask Seller. Just how many cat owners had their lives hijacked by their own pet cat because the latter wanted to live longer?
Thanks to the events of the letter reading and rejection, you realise that Muge is rendered terminally suicidal. If it wasn't for the whole stealing humanity issue (which isn't much better and is practically allegorical to her situation), she could've killed herself right there and then.
The Mask Seller isn't stopped for good. There is nothing stopping him from simply going to the next depressed person after his plans in here are foiled. Given that this is Japan we are talking about...
