Was the marriage valid or void, and what is the ruling on marrying a thrice-divorced woman in the event that the contract is void, and does sin result from that?

If the marriage guardian (wali) is a Muslim, the marriage is valid, even if he is a dissolute person (fasiq) or an innovator (sahib bid'ah) whose innovation does not amount to disbelief (kufr). However, if the innovation amounts to disbelief, the contract is invalid according to the Shafi'is and Hanbalis, and you bear no sin if you were ignorant of this ruling.

The divorce issued by the husband is considered valid. You are not permitted to return to your wife, from whom you have been irrevocably divorced (baynunah kubra), unless she marries another man out of genuine desire, he consummates the marriage with her, and then divorces her or dies, and her waiting period ('iddah) expires. It is not permissible for someone who believed the marriage to be valid to subsequently base a ruling on its invalidity that aligns with his desire after the divorce.