Is it permissible for the authorized marriage official to change and predate the marriage contract from its actual date, and does this invalidate the contract?

If a marriage contract fulfills its pillars and conditions, then the contract is legally valid. An error in the date of writing and officially documenting the contract does not affect its validity, because the validity of the contract depends on its pillars and conditions, not on its writing. Writing and documenting the contract are required to preserve rights and prevent disputes. It is the right of the spouses to consult the competent authorities to correct an error in the contract's date if it leads to the loss of rights.