Thanks for the answers!
I'm not sure how it all works, but I'm guessing it would involve interpolation of some sort. Imagine an object moving; the physics engine steps, updating the object's position, then the render engine renders the object halfway in between the new position, and then all the way, before the physics engine updates again. I guess that might not be good because the render engine would be behind the physics engine. It would also probably be very complicated with different ratios of render/physics frame rate. Also, I'm not sure how Spine works exactly, but if I used Spine models, wouldn't it be possible to interpolate the animations to render at a higher framerate than the physics engine? It's probably more trouble than it's worth, but it doesn't hurt to know.
I understand if you don't want to take the time, but I would definitely benefit from an example of one of your wrappers that you do. I'm still a novice programmer so I don't have a solid grasp on that kind of thing.
And I'll look into the things you mentioned. Inheritance is probably a good option, I'm not used to dealing with it so I tend to shy away from it. I guess it's time I learn.