It was Targett and not Davis. He had to move across from the left to try and cover but was overloaded by the two or three Everton players coming at him. He takes a step to his left just before deciding that he needs to get across to Lukaku which leaves him a fraction late to make the tackle. There is only a fraction of a second at this level to decide what to do and he needs more experience so that his positioning becomes instinctive. This move came direct from one of our corners which was poorly delivered (Davis?).
I agree with the rest of your analysis.