I’ve not seen that tutorial through before but skipping through it it looks like everything is based on the positions of those corner nulls. And you’re saying that when you try to move them, they keep springing back which makes me think that there is probably an expression or a keyframe somewhere on the nulls’ positions which is setting them to the original place.
Have I understood correctly? Could you tell me how far through the tutorial you’ve got? It might be easier to work out what’s going wrong.