What is best for the Fed tax filing? Dependent pays more than 1/2 living expenses'...or not? (My 15 year old on SSI, receives more if she does contribute)

Social Security or ssi or SSDI doesn't count as support.  If she is under 19 you can claim her no matter how much income she has.

You can claim a child up to the age of 19 (24 if they remain in school) no matter how much  their income is and I highly doubt she pays more than 1/2 of her  total living expenses (don't forget the housing/meals/medical/etc that the parent pays) so the support is a non issue. 

Thank you