The data is polymorphic, not the behaviour.

Same thing.

Look at how much work you're doing your way. Look at how little magic you need to do it my way.

Encapsulation, object-wise, is hiding data behind polymorphic behavior. I don't know how to say it any more plainly than this. Mixing raw data access and polymorphism usually bites me, and I think the fragility of your solution here is evidence that it's already biting you.