If you have a leak in the seal of the lid, as fermentation slows the CO2 will go out there and not through the airlock. The water in the airlock provides more resistance for the air.
It may well still be going.
Different temperatures, yeast amount and health, different amount of aeration can all affect fermentation time. Nothing to worry about.
You know it started to ferment, so it has made alcohol. The alcohol will protect the beer. Relax. Move away from the fermenter and don't touch it for about 2 weeks (or longer if you can manage it). Do some reading on the site, there is no issue with leaving the beer on the yeast for a while, and it will actually help the beer.