Answer: It's normal for citrus trees to drop leaves in the winter time, but I think the excessive drop of leaves from your tree indicates some water stress. Indoor air, especially in winter, can be quite dry. It would help your tree if you misted the leaves each morning, or if you placed the container on a gravel-filled tray and kept some water in the tray. Not enough to keep the roots soggy, but enough to evaporate during the day and increase the humidity around your tree. Citrus trees like moist but not soggy soil. You may need to water more frequently than you're doing right now. I'd let the top half-inch of soil dry out and then water thoroughly. Fruit drop can also indicate water stress, but citrus will sometimes develop more flowers than it can support and as a result will drop immature fruit. Also, the fruit may be aborting because it was not pollinated. You can help things along by visiting each open flower with a small artist's brush, effectively transferring pollen from one flower to the next.
Scarring on the rinds indicates the skin was scraped or scratched as it was developing. The scars will get more pronounced as the fruit grows larger. The damage may have occured before you even got the tree, so I wouldn't worry too much about it. The fruit inside should be just fin.
Hope this information helps you help your tree regain its health!
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