will seedlings survive with just natural light?

They don't need a humidifier, save your money. Cannabis is not a rain forest plant, but a (mainly) lowland dry season plant, growing with minimal humidity. Sunlight is how cannabis has been growing since forever, they will be fine. Glass will stop UV light, so putting them in direct sun would be better for growth. 15 hours will be fine, they need to get to a certain age before being able to flower. Next grow, I would suggest filling your pots right up to the top. This gives vastly more room for root growth, plus your plants will get a lot more reflected side light. As can be seen in the Northern Light Day 2 picture, how much shadow this plant is in and how little light gets to it by being so deep down in the pot. Being so deep down can cause excessive stretching. Hope this helps, Organoman.