You've really not provided enough facts to give a good answer. A lot will depend upon which state issued the guardianship (CA or NV?) though I suspect they will be pretty similar. In Nevada, the guardian has first say in what happens with the child, and Dad would have to terminate the guardianship before he makes those decisions unilaterally. And early emancipation requires a legal proceeding, so merely signing a piece of paper will not divest the guardian of her ward. As for the Social Security Benefits, it will depend upon the type of benefits she receives (and the basis for them) so she should contact the Social Security Office directly for that answer.
Mr. Pickard's answer is absolutely correct. But I am not sure what you mean by temporary guardianship. If there is no court order, dad can void a temp guard. But there also must be grounds to emancipate, and the minor must have the ability to support her/himself.
The social security checks will continue to come in until she turns 18. As to the other part of your question, you need to consult with an attorney who can examine all the facts and give you proper advise.
Sign up to receive a 3-part series of useful information and advice about child custody law.