You might as well buy a new sensor and check the OHMS on the old and new one. You can see if the old one is within the same OHMS range as the new one. If not, then replace it.
Yes, it's possible. It can be tricky to find the leak unless you have a fog machine or you can take it to a shop. It's possible that smog shops has one.
Charcoal-filled canister's job is to absorb fuel vapor that would otherwise vent out to the atmosphere, causing pollution. Vapors trapped by the charcoal are released back into the engine through the purge valve and then burned.
