1. you have to treat all income the same. Government child payment is still income, if you discount that it's income-source discrimination.
2. disablility cannot be discriminated.
3. Your only valid reason is that you found someone with higher income/credit. So until you find that person, you are kind of stuck with her.
You have to let her apply, but you can let her wait for a few days as you wait for other applicants.