To me, it sounds like you may be complicating your setup unnecessarily. There are other approaches to mouth animation that might be less work intensive. It really depends on how you intend to animate this. Based on what you written so far, here are some suggestions:
1. Set up a Smart Bone Dial action and create all your mouth poses using one set of artwork in a single animation--don't bother with a switch layer. Separate each animation by maybe 10 frames to make it easier to stop at each animation on the SBD. When you animate, used Step mode for the SBD (suggest using Use Copy Previous Key so this doesn't interfere with other keyframing modes.) When animating the mouth, you can simply dial to the mouth shape you want. Since you're not interested in interpolation, this should just snap to each keyframed pose.
The nice thing about this method is that it's also very easy to add more poses, even with nested groups and multiple masks, without affecting poses you may have already keyframed (assuming you're not going to obsess about the order of the poses.)
2. Use Morph Actions to save presets for all your mouth poses, and set them up with Step frames to prevent interpolation. Name the poses with a clear prefix and description, like 'Mouth.Neutral', 'Mouth.Open', 'Mouth.Sad', etc. Now, when animating, just select the pose you want from Actions and insert the pose.
If you're using Moho 12 (as opposed to ASP), the nice thing is that you can finally save your Actions as a file and import them to other scenes. So, for example, when you create a new mouth pose while animating a scene, you can now easily import the new pose to your master character rig project file to use in your next scene.
3. Personally, I usually just copy the pose group that's closest to the next pose I wish to create in a switch layer, masks and all, and edit that art as a new pose, and move it the end of the list so it doesn't affect any existing animations. Most of the time, I prefer to control this with an SBD set to Step, and Interpolation set to Copy Previous Key...this way, nearly all my character animation keyframes appear in a single bone layer,
including the Switch Layer animations. I find this easier than drilling down into the layers hierarchy for Switch Layer keyframes. It's a little more work to update the Smart Bone Actions but creating the artwork itself can be pretty quick and easy. And once it's set up, it's very fast for lipsync animation.
For 'specials', I may just edit an existing pose on the timeline and save that pose as a Regular or Morph Action to reuse later. I usually won't bother adding this edited pose as a new Switch Layer drawing if it's not a pose that will be frequently re-used.
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I don't know if any of these approaches helps you but maybe this will help you consider other, possibly more efficient options. There are countless ways to set up and animate mouths in Moho, and some methods can work much better than others depending on the production's designs and animation style.
In my experience, when I find something I'm doing is getting hard to do, then I'm
probably doing it the hard way.
