I think we are not confused. The I-864 is an agreement between you and the US government which stipulates that you will reimburse the government for any benefits she uses.
So, your I-864 is unrelated to the current divorce support agreement. To terminate the agreement, between you and the US government, convince her to naturalize. Good luck.
Income claimed on a 1040 is not the same as Social Security Administration work credit.
The IRS and Social Security Administration are different agencies.
You are obligated until one of the following happens:
1. She becomes a US citizen.
2. She accumulated 40 quarters of work credit.
3. She leaves the US and loses her legal staus.
4. Dies.
It is in your best interest that she becomes US citizen ASAP.
Terminology is important in immigration. Using the term "wife" implies marriage. The CO needs only suspicion that a secret marriage has taken place to immediately disqualify a couple from a K-1. It would a shame to get a denial at the interview stage. @JeanneAdil is pretty familiar with MENA countries and their culture.