On the light issue, you are probably correct. I have personally seen leaf burn from light through windows though.
My Celtis Africana drops its leaves when I move it to another spot in the garden, and then grows new leaves.
Humidity needs to be 50% and above, with appropriate air movement. 60-65 F will be good for the tree.
Gardenista is a site that has an article on growing a lemon tree indoors (Or at least letting it survive until spring), have a look. It might be useful.
www.gardenista.com/posts/5-secrets-tips-...-indoor-citrus-trees
If you have success please let me know what you did.