Two questions: Masking and Warped Art

I am syncing mouths and am running into two issues. My character is designed so that instead of there being a jaw that goes up and down in sync to the mouths, the mouths instead are masked on the bottom of the head shape and are cut off on the bottom when they reach beyond it, like they remain inside the head. He's made to be animated in Adobe Animate, but I can't figure out how to do that in Ch. Is there a way to mask assets instead another layer asset?

Secondly, I notice that when I put a smiling and frowning mouth set inside a Trigger Swap, the latched set appeared to be warped in the middle of their artwork when they're recorded. What is causing this?

Thank you!