Scheffs also like a bright spot indoors with a few hours of direct sun. If you are keeping it in the corner like your image shows, and misting it, you could be the cause of your own trouble.
Growing a schefflera indoors with 40, 60 or 70% humidity will not make much of a difference.
If you want to increase humidity, the tray with pebbles is a good idea. If you are using a wood stove for heat, you could leave a pot of water on top to release water vapor into the air to increase humidity.
Misting is rarely a good substitution for good soil, a good container or good light. It is just a small part of the overall health of a plant!