I'm a beginner, and never had to deal with it personally, but from what I've read here's my advice and someone may come along and correct me.
It sounds like the tree was stressed, and then maybe over-fertilized, and then stressed more by soil getting dry, but when you drench the soil, and then keep the pot in water that can also stress the tree. and possibly cause root rot.
Don't let it sit in water. Maybe put some pebbles on the plate with water and place the pot where the water is not actually touching the pot, for humidity.
Place your tree where it gets enough light, with proper temerature/environment and LEAVE IT ALONE for a while. Only water when soil is dry...
I hope this helps. And as I said, I've never dealt with this personally, just a little info I've gathered so far