Well, if the magnets that attach the mask to the head assembly aren’t the ONLY magnets in the head or in components of the masks, I see no reason complex articulation couldn’t take place with magnets on actuators in other parts of the face operating sliding plates on the surface of the mask to convey facial expressions.
I believe the main reason to the masks attaching to the mouth is due to the sets. The Mask of Life’s retractable mouthplate is non-canon. (However I don’t know if Gresh’s and Tarix’s is)
You are right, as that’s the easiest way to attach them in the sets. But it doesn’t have to be the way they attach in-story. Whether the masks in the movies are canonical or not, they offer one potential way of visualizing the way Kanohi could attach. The canonocity of the movies is also a debateable subject, as we’ve been told multiple times that they’re either the most canon media, the least canon, or somewhere in between.
Either way, this is a small detail–I’m not sure that it needs a ton of elaboration, but I think that the various media we have makes a case for Matoran having some form of a moving mouth.