It looks like you are making a mistake in computing the difference in both cases. Here’s what I get as the answer for Ex3:
Your backward propagation works perfectly fine! difference = 2.919335883291695e-10
Notice that in both cases, your answer is about the square of the correct answer at least in terms of order of magnitude. BTW in Ex4, I assume you had already fixed the intentional bugs that they put into the back prop function.
I suggest you carefully compare your implementation of the difference calculation to the math formulas given in the assignment and the lecture. Are you sure you didn’t use the square of the norm in the numerator?