Is the marriage contract considered void, and if so, what is the solution, given that the following errors occurred: the contract was written by a woman, the legal guardian was absent, and the full dowry amount was not mentioned?

Documenting the marriage contract is not a condition for its validity; rather, the reliance is on the contract between the guardian or his agent and the husband or his agent. So, if the contract was concluded between you and your wife's cousin with authorization from her father, and the offer and acceptance took place in the presence of two Muslim witnesses, then the contract is valid. The failure to mention the full mahr amount does not affect the contract's validity. However, if the cousin arranged the marriage without authorization, or if the contract was concluded without two Muslim witnesses, then there is a difference of opinion among scholars regarding the contract's validity. In the event that the contract is not valid, it can be renewed through offer and acceptance between the husband and the wife's guardian in the presence of two Muslim witnesses.