Some pretty spot-on analysis above, re camera's just a bit too low, a slight double chin in #1. For the butterfly shadow placement, the light needs to be right over the lens axis, so a boom arm is pretty handy. As jowens pointed out, for the jaw thrust it is out and down...in #2 she's pushing her jaw out, and up, which gives an unflattering up-nose angle, which combined with the camera-to-subject distance and lens focal length, makes her chin look a bit too prominent, and as stated, we see up the ole nostrils. Alternately, she could be told to sit up super-straights and tall, and then lean forward from the waist, and then lift the chin just slightly...this adds a feeling of movement, of vitality, and is slimming to chubby people (which she is not). This achieves the "down" part by forward lean, and when the chin is lifted slightly it goes "out", and away from the body, and improves the line of the neck and under-jaw areas. In shot #1 she is just...sitiing..there..upright...zero feeling of movement or dynamism...in #2 she has some head tilt, so #2 has more "movement".
Posing is both art and science. A few things stand out. In #1 some slight wrinkles under her eyes show up as light gray circles; to eliminate those I'd grab the healing brush and make a slightly irregularly-edged, crescent-shaped selection and just 'heal' them, a la Doug Gordon's method (so much faster than brushing in or cloning with soft brush at medium opacity and flow). In #2, the under-eye color is not gray and the slight wrinkles she has appear less-prominent. So, you can see a difference in the eye are from #1 to #2. I think the under-eye and edge-of-the-eyes areas are where most women see their "age" and either like/dislike portraits of themselves. Kinda like the beer gut on guys...if that looks good, it's "OKAY!"
I think an under-chin reflector would have helped both of these shots. Just out of frame, and angled up at at her face, a reflector about the size of a large cardboard pizza box, opened up, white-side up, and held by her, or placed on a tilting stand, or table.