Your best bet is to use an instance verifier instead of, or in addition to, the property level verification you're already doing. When verification of a property is dependent on the values of other properties instance-level verification of the entire entity is usually preferable.
As for how to force the setter to fire, I really don't know. The default DataSourceUpdateMode of the BindingDescriptors is "OnValidation", which means a change has to be made and the control exited before the setter is called. The "OnPropertyChanged" mode, when set, means that any change typed into the control causes the setter to be called. But there's no mode available to call the setter just when tabbing through. You'd probably need to hook into control or form events to do something programmatically. But, instance verification would probably be a much cleaner and easier approach.