1. I don't believe this is legal
2. he is supposed to give you written notice of the security deposit
3. This will depend on what YOUR lease says. The typical/standard lease from the Real Estate Board says that you are responsible for the rent until the end of the lease period. The landlord is responsible for trying to find another tenant. You are responsible for any costs he incurs in trying to rerent it (realtor commissions, advertising costs etc). He is not allowed to collect double rent so if for example, you paid October rent and he finds someone for October 15, he owes you 1/2 months rent. Again, this depends on what it says in your agreement.
It would be helpful to consult an attorney unless you can come to a mutual agreement with your landlord.