Sometimes there is confusion about the difference between permanent residency and citizenship. I think you would do well to look up the definition of both.
Whatever civil attorneys can recover in court is NOT related to the contract between the USC who signed the form and the United States. You're splitting hairs and comparing apples to oranges.
The OP of this thread saying that the I-864 obligated him to pay a specified monthly payment is incorrect and was either gravely misled or failed to do his own homework.
You are not obligated to support her. You signed a contract with the US government agreeing to repay any means-tested benefits she used without being eligible.