Yes, the point to point tickets work in both directions, but even the French are sometimes stupid about this. If you're making a round trip to the airport or Versailles or something, you should buy two tickets immediately before you leave. You cannot imagine the lines of people at the ticket machines in Versailles who didn't think to buy their return ticket ahead of time.

In Paris, nothing matters -- you can go from anywhere in zone 1 to anywhere else in zone 1 with an ordinary metro ticket (T+).