Is the contract considered legally valid if the owner's private lawyer is the sole witness to it, and her signature is deemed sufficient and admissible in court according to local custom?

Witnessing financial contracts is desirable, not obligatory, and is not a condition for the validity of the contract. Rather, it is for securing and guaranteeing rights. If security is achieved through the signature of a law firm, then there is no harm. The contract is valid with its Sharia-compliant conditions even if it has not been witnessed, based on the Almighty's saying: "But if one of you entrusts another, then let him who is entrusted discharge his trust and let him fear Allah, his Lord." Ibn al-Arabi conveyed the consensus of jurists that the command to write, witness, and mortgage is a recommendation and guidance for precaution, not an obligation.