Is the marriage contract considered valid if concluded without the guardian's knowledge or consultation, and are there additional conditions required of the wife in this case?

If you authorize your brother to conduct the marriage (nikah) and he is present at the عقد (contract) and accepts it, then the contract is valid.

The contract is also valid if the authorized marriage official (ma'dhun) initiates it without your knowledge or the knowledge of your representative, and then you or your representative accept it.

Thus, the marriage is valid in both cases.