Would my signature on a handwritten informal marriage contract for a previously married bride whose guardian was not present constitute a sin, and would that affect the validity of the marriage?

Marriage without a guardian (wali) is invalid according to the majority of scholars. However, if the marriage contract was concluded believing in its validity, following the Hanafi school of thought, then the marriage is valid. If you testify to the contract, believing in its validity, there is no sin upon you. But if you believe it to be invalid, there is a difference of opinion regarding the ruling on your testimony: some scholars permit it because the contract is a matter of scholarly disagreement, while others do not. It is permissible to testify to a flawed contract that is a matter of disagreement if the one performing it is acting upon an interpretation (mu'awwil) or imitating an interpreter. However, if there is a consensus on its invalidity, then it is not permissible. This is coupled with emphasizing the importance of documenting the marriage contract in courts to protect rights.