What can I do to correct my nonexistent jaw / protruding chin / facial lines associated with flatness of features?

Thank you for your pictures. In my opinion you could benefit from some cheek implants, sliding genioplasty (setback) and some liposuction beneath the chin.  This would improve your facial balance.Best wishes.

I perform regularly all kinds of genioplasties, not only sliding
genioplasties but also one, two and three-dimensional augmentation
and/or reduction genioplasties, plus other custom designs; in other
words, I know well what I am talking about.
Your case is the among the best you can meet doing chin surgery to perform a three-dimensional osseous reduction.
That is the name of the procedure you are an optimal case for, since you need: massive vertical reduction, important horizontal narrowing and a moderate anterior deprojection/reduction of your chin dimensions.
This is an awesome, lifetime lating, stable and safe procedure, which notwithstanding needs very accurate preoperative antropometric planning, desing, discussion and a brilliant execution, a technique full of creativity and good aesthetic judgement requirements.
Additionally I'd sugges rhinoplasty, specially to raise the low radix and make a better straight profile; other enhancements on the nose as per your request.
Feel free to request any additional information from me.

You upper jaw might be back slightly as well. You would need to have X-rays taken to see where the problem is. Surgery is indicated.