What are a tenant’s rights regarding an early lease termination?

Generally, the lease will control all duties, liabilities, and rights of the landlord and the tenant.  He should have honored the month-to-month set out in the lease and allowed you to give the required notice and move out when you wanted to.  Unfortunately, you signed a new six month lease-- so that lease will guide your basic rights.  You will be liable for the balance of the lease as provided by the contract if you break the lease and leave early.  However, you do have what is called equitable relief-- which generally refers to rules of fair dealing.  Even though you would face paying the balance of the lease until it's re-rented, the landlord still has a duty to mitigate damages by actively looking for a new tenant.  He can't just sit and collect rent from you after you leave the unit.  Considering that he has known for a while that you were trying to leave and forced you to sign an agreement in violation of the first lease, you could file a small claims suit against him for failing to mitigate damages-- in fact, it appears his actions have increased the amount of damages (i.e. back rent) that you would owe.  The court may or may not grant a motion to have the second lease declared void, but it's worth the effort to combine it with your claim against the landlord for failing to mitigate.

If you signed a six month lease, you are obligated to it for the full six months unless the lease itself gives you the right to terminate it early (e.g. on 30 days notice) and you comply with the early termination requirements. Otherwise, you will have to pay until the end of the written six month term, though not beyond that. It doesn't matter what you and the landlord may have orally discussed prior to signing the six month lease; once you sign a written lease, it supercedes any prior oral discussions (for future reference, make sure any terms you discussed orally get into the lease or contract before you sign it).