First of all, your little ficus looks nice and healthy. And you're doing research and going on the forums to look for understanding, which is great. However, you've found one of the problems with researching, which is that most information sites, books or online, have been written by people drawing their information from other written sources, which also drew from written sources, and so on and so on.
The forums, on the other hand, are composed of people who grow plants and share their experiences. I've been a professional interior landscaper for 30 years, and I've taken care of thousands and thousands of ficus trees, and I can tell you that without a doubt, if you have a ficus that came from a grower a couple of weeks ago, it drops leaves because the light has been changed. It had been growing
in fairly high light, then it goes to a store where the light is much reduced, and it tries to adjust by dropping unneeded leaves. Then you get it, the light goes up again, and it doesn't know what to think. By the way, either of the places you've been keeping it will provide good light for a ficus, just leave it in one or the other so it can adjust.
So here's the first thing: leaf drop in a new ficus is to be expected, and is caused by reduction of light. The 2 leaves that are yellow are in the process of being dropped, not only because of the light changes but also because they are old. 70% of the plant's work is done by 30% of the new leaves. (Or something like that, don't quote me on numbers).
Here's some more stuff about the things you mentioned. Most important, and a thing many people don't know, is that you shouldn't repot a new plant. It's already in good planting medium (a soilless mix is what growers and interior landscapers use because its drainage properties are superior to potting soils such as MG). Since it has also been heavily fertilized to promote growth, new plants should not be fertilized for 6 months, either. Over fertilizing is much more of a danger than under fertilizing.
Another concern is up-potting. Unless the ficus was so big that it was toppling over, or the pot was completely solid with roots, it didn't need to be up-potted. When it gets so big that it won't stand up, or has so many roots that there's no room for water, and if you want it to grow bigger, then you can up-pot, but only one size at a time, e.g., 6" to 8" (measuring diameter at top of pot), 8" to 10", etc. (If you want it to stay smaller, instead of up-potting, you can root prune, and repot back into the same pot.)
I would recommend you put it back into a smaller pot, because having that large area of damp soil around the outside of the root mass is really asking for root rot to set in.
Something else you mentioned is tap water. One reads lots of talk of water, and how bad tap water is. I'll just say this about that. For all the hundreds of thousands of times I took care of plants, I always used tap water. Now there are probably some kinds of plants that need special care, but the plants commonly used in the house are tough and adaptable, and tap water doesn't hurt them Unless the water is softened, then the salt level is unacceptable and you would need to use rain water or bottled water. But I have read that distilled water is not good for plants.
This has turned into a really long post, but you brought up so many common misconceptions that I really wanted to address. Of course there's lots more, like watering and bugs and pruning...I invite you to stop by my blog, The Ficus Wrangler, where you can ask any question you want.
Hope to see you there.