If they've stopped dropping leaves and appear to be recovering, I'd be slow to change anything I was doing as it really may just have been the adjustment. Although, you usually don't see this type of thing (rapid response and recovery) with the species mentioned. North facing windows are not great for light loving plants. But if that's the space you have to work with, then that's what you'll have to work with.

The plants should be fine, they'll adjust and grow more slowly, but the species mentioned should not have long-term problems in their location.
Earwigs, do you mean? If they're in the pot, that would probably be a sign of too much moisture. If you meant earthworms, bonsai pots are too small and shallow to support them.
The fruit of the Barbados Cherry is not poisonous, as far as I know, but I would not exactly consider it edible either. To thicken branches or trunks, the principle is the same, allow the plant to grow out before pruning. For the Barbados Cherry, I'd allow it to grow out for an entire year before pruning. Of course, you do want to still remove unwanted branches, back buds, etc.