I’m trying to set up some elastic collisions using Sprite Kit
I think you’re misunderstanding the physics in play here. An elastic collision is an oversimplification of a collision. In reality, no collision results in a perfect transfer of energy. I’ve explained the physics here in an answer to the related question you linked to.
I suspect reading through that you will see that an “elastic” collision can be done in SpriteKit, but most people don’t realize you need space between the objects. You shouldn’t need to do your own physics calculations.