"Please... thoughts?? Do I owe this woman something?"
Although I can't speak to the business side of cakes, I can offer my personal viewpoint from the customer perspective.
If I had been a super picky customer, (personally, I'm a really laid-back customer), and had expected to see THAT EXACT CAKE, I would have been disappointed as well. It appears as though you forgot the belt, since the belt loops were incorporated in the design of the cake photo provided by your client.
"I'm really not sure if I owe her much since I already bent over backwards for her. I do admit the cake probably should have incorporated a belt, but I guess I just see it as [not?] that important for the overall design."
Yes, your cake was beautifully executed, but you were already aware that the customer was "super-picky" and did not copy her requested design, exactly. There was no stand-up numeral topper, nor any daisies.
The customer probably doesn't care if you "bent over backwards", or even if she got a great deal on the pricing. She was very specific on what she wanted, and didn't get it.
Perhaps a $40-$50 refund, and a very valuable lesson learned. I suspect that the unhappy, "super-picky" customer would be insulted by a $20 refund. [Disclaimer: I'm a hobby baker and do not deal with cake customers. (Thank goodness!!!) I did, however, deal with a lot of picky, demanding, customers for 30 years before retirement.]