I think the more I alter the fabric, the more issues I am coming up with. I should have just listened to you and started over in the right size. That's what I am going to end up doing. I'll take a look at the Simplicity version, which I believe is the licensed S&S stays pattern.
I am super glad you came to that conclusion. I've been thinking of how to help you over the next steps...but they are giant ones and too hard to explain via internet. (Redoing the straps means basically redrawing the front of the arm scye, which you could explain if you were close, but you aren't close yet.) If you get the right size to start with, the straps are more likely to fit as-is, or close.
Take this rough muslin apart (marking seam lines with a marker), and see how it compares to the size that should fit you. If there is a big difference, do the standard piece, and do another piece in the size closest to your muslin piece. Start by using standard, but adjust as needed using the custom sizing. (you may end up mixing and matching sizes a bit). Give yourself big seam allowances (1 inch.) trim these off as soon as soon as you are sure you don't need them to standard SA (5/8 or recommended.) Don't jump into making the real thing, you still first need a finished working muslin that really fits you. You've learned a lot so far. Keep it up!