What else have you done to increase humidity? Don't spray the plant (waste of time) but do either use a room humidifier or set pots of water around the tree - one pot on the heater may not be enough. You haven't mentioned if it gets decent light - not in a south window necessarily, but it is a tropical and needs good light all day. I'd lengthen the time between waterings for now, though do it well when you do it, but have you also considered that it might be rootbound and need repotting?

If the branches are as brittle as you say, that's not good. See what the nursery has to say. I'm sure others here will have some advice for you too.

Can you check the soil moisture around the base of the main trunk, notplantsavvy? It could be that the soil around the root ball has dried out. In cases of soil moisture issues like that, the leaves begin to dry out from the edges and brown out completely. It happened to me two winters ago (shrub was planted in the ground though) due to a sprinkler system that was left turned off too long in the middle of a dry winter. Make sure that the area around the main trunk receives sufficient moisture. On the other hand, too much water can also cause similar symptoms specially if the soil does not drain well, if the pot has insufficient drainage holes or if the pot is allowed to sit in water. In this case, the roots slowly develop a fungal infections called root rot and are unable to provide the plant above the ground with the amount of moisture that the shrub needs. The plant browns out just as if the plant was not receiving enough water because, in a way, it was not. If you examine the roots of these plants, you will notice that the roots are brown. I assume that you have no pest issues with this camellia because you reported no damage to the top/bottom of leaves, no damage (cuts, etc) to the trunk and things like that. Luis

My guess is that it is severely root bound and needs to have the roots soaked. Also, it was probably grown in a greenhouse and used to high humidity. You will get a better answer on the houseplant forum, we talk about the edible fig here.