Indoor and greenhouse figs tend to drop leaves when light level changes or the plant is moved. If you have the time or want to take the effort, you can turn it just a little each day and avoid leaf drop. I would absolutely put in a larger pot. The stalk or truck can't regrow leaves from the leaf nodes but they are likely to for a branch there if cut back hard. Esp in the spring.

Once leaf loss is underway, it's not unusual for the symptom to get worse, even while the cause of the symptom becomes less limiting as you're get watering under control. The reason is, once the shedding process reaches a critical point, it can't be reversed, so any leaves that already have/had abscission layers forming where the leaf is attached to the stem are destined to be shed. That sort of makes your job out to be getting the plant through the winter and into a state of vitality that will allow the plant to be repotted and subsequently pruned back. Once you do that and get the plant into a soil you can water correctly, you'll have a much different scenario. I various ways, I often point out that when using a water-retentive soil, you end up battling the medium for control of the plant's vitality. That's frustrating and no where near as rewarding as growing CAN be. Keep in mind that the 6-7 months you should wait to do the work is little more than a blink of an eye to a tree. This might sound corny or new agey, but if you settle yourself into the same rhythm the plants go through in their annual growth cycle so you can take advantage of their strengths and allow for their weaker times, you'll feel a much greater sense of unity with your plants. More knowledge, a better understanding of how plants work, and the confidence that comes from knowing you are acting with the best interest of the plant in mind (which can only increase your own gratification), all act to elevate the plane on which you interact with your plants. If you pare the reason you're here down to its bare bones, isn't it because you want more from the growing experience? Al

If this is the only room available, how about putting it directly in front of the big window with the blind pulled right up, pot raised high enough so all the leaves are above the level of the sill, so it's in between the window & the grow light? It still may not be enough light, but maybe worth a try. That doesn't address the brown spots, but I doubt there's any sunburn, if that's the place it's been in the whole time.

As I said in your other post: If it’s not growing, lack of light is probably one of the main factors (there can be many others, but let’s focus on this first). People may think their plant get lots of light as it seems bright to them, but the plant usually doesn’t see it that way. Right up in front of a southern facing window (if in the northern hemisphere) that’s unobstructed by blinds, trees outside, buildings, etc, would be the best spot. Yes, receiving “direct” sun. That sun is being filtered by the glass and is actually not direct. Your only concern would be if you have no airflow in your house. Heat from the sun can be an issue. The smallest of fans keeping the air moving in the room is all you’d need. This isn’t the best time of year to hope for good looking growth. While these plants never go dormant and will grow all year, the days are getting shorter and the sun is growing weaker. Your tree will put out its best growth from about april-August.