Hello.

I got a bonsai tree about a month ago, which I think is a Chinese elm bonsai.
Apparently, the seller suggested to water it once every single day, which I figured was not ideal. They didn't give much more information on it. I watered it approximately 4 times/week in the first two weeks, which I thought was too much but I hoped it would be a good compromise. Now I water it about twice a week.
This is usually the scenario: the end of a leaf starts to turn brown and it's got this weird, almost like "crispy" texture. As the browning continues, the leaf falls.
The tree also has tiny brown dots on some of the leaves, which it has had since I got it.
I thought it might be improper watering, but I don't really know. I don't know much about bonsais at all, I just didn't want to lose this one. I also saw the "big debate" whether such trees should even be kept inside, so maybe that's the reason, too, as I do keep it inside. The air where I keep it is fine, I think, not too dry, but I do sometime spray the leaves with water to be sure. It is not getting much sun now, in the winter (because of this strange orientation of the windows).
I also changed the soil because mold was starting to show up only on the top of the soil (more specifically, on top of the moss layer which was there initially, when I got it), and I thought the soil might have other parasites in it, but that didn't help much either. I can't see parasites on the leaves or branches either.
I'm attaching some pictures of the tree and those tiny black dots.
If anyone has any suggestions as to what might be going on or how I can save the tree (if it's even possible at this point), please share your knowledge with me.
Thank you!

First you never water to a schedule, water when it is needed. Next it should not be inside, there is nowhere near enough light and it is not cold enough, it will slowly die. Next never re pot a weak tree, there was no need to do it you were overwatering. Of you cannot put it outside then put it in a sunny window in as cold a room as possible. No heat. You may be lucky and it survives.

Hello, this is not a Chinese Elm... Looks more like a Duranta Repens.