It seems pretty clear, if your not flying then your companion can't fly, you risk cancelling your companion pass and/or getting your companion stuck somewhere. I would not risk it. You are already getting a free flight from AUS-OAK and only you can decide whether the OAK-AUS flight for your companion is worth the price or convenience of just flying from SFO-YYZ. Per Southwest Website regarding companion pass

The Companion Pass allows the Member to designate an individual to fly with the Member (such designated individual, the “Companion”) on Southwest Airlines flights purchased by the Member and booked through Southwest Airlines during the Companion Pass validity period. A Companion's reservation is not considered a purchased flight, and therefore, another Companion Pass reservation cannot be booked from it.