Hi! It sounds like you've already got good sense about the plant. Maybe it just needs more time to recover from the soggy soil, before the leaf drop stops.
With all due respect to plantladylin, and it is a lot of respect, I don't think a tray of pebbles is going to do much for humidity around a big plant like that.(from what I read from tapla). Someone else here has had a fiddle leaf for many years, and I don't think she puts any effort into humidity, other than keeping hundreds of other plants big and small all through her house
