Welcome to Tropical Fish Keeping forum.
I moved your post out of the very old thread in which it was posted, and started a new thread of its own. It is best not to post in old threads, or those not directly unrelated to your specific issue.
Now to your questions. Winter sunlight for those of us in the temperate zones is significantly less than during our summer period. However, you are in Nepal, much closer to the equator, so this is less of an issue.
Generally speaking, ambient daylight/sunlight entering a room will not be sufficient for plants in an aquarium unless the tank is basically in front of a bright window. But then the plants will grow to the light, which makes viewing them difficult, and they grow "odd."
Overhead tank light is best, as it allows the plants to naturally grow upward which is not only better for the plants but for us viewing the aquarium.
I have never experimented with light under the water, but my first issue would be it is totally un-natural for not only plants but fish. And this could cause serious problems for fish health and plant growth. I would keep the tank lighting over the tank.