1) Not a tax question. Maybe you should ask on "alt.social-security.disability"....
2) SSDI is the same regardless of whether the person is a dependent or not.
3) SSI component of SSDI may be affected. That item is needs-based.
4) If there's a tax payback for having including it in income, see "claim-of-right" (tax term).
Tax issue: Are you certain that the daughter is a dependent? Since she receives SSDI, she is obviously an "adult child." Disability is not an exception to the "gross income" < "exemption amount" test. SSDI is includible in income to the same extent as SSRB, so it's possible that it might have been subject to tax, especially if there were other income.