Orthonormal, you're rehashing things I've covered in the post. Yes, many reasonable discounting methods (like exponential discounting in the "proximity argument") do have a specific step where the derivative becomes negative.

What's more, that fact doesn't look especially unintuitive once you zoom in on it; do the math and see. For example, in the proximity argument the step involves the additional people suffering so far away from you that even an infinity of them sums up to less than e.g. one close relative of yours. Not so unrealistic for everyday humans, is it?