Since we haven't made mouth shapes for the various phonemes and we're not using a voice track, we don't really need to worry about matching the mouth movements to the words. Really, all you need to do is animate the mouth opening and closing, with appropriate pauses for the switches in dialogue (where the text bubbles switch from one to the next.)
For this you don't need to adjust any pivot points; just make key frames and animate the upper lip moving up, the lower lip moving down; if you made the "mouth interior" layer underneath, you should be able to see the inside of your character's mouth as he/she "speaks". You should be able to do a full open and close using just five frames; then copy those frames over and over again for the length of your animation with a pause here and there.
