Two problems (unique to going slow, my first example actually didn't work completely right either, it was just hidden by going fast).
1. You didn't turn off using default controls. Jump overrides the move up one pixel event (you jump first, and then move up), so you pick up all the momentum from your jump when climbing the ladder, which is way faster than what you want. With simulate controls, you can put the jump event AFTER the move up one pixel event - you move up one pixel first, and are no longer on the floor, so you can no longer trigger the jump.
2. Having two ladder events using else - the second else runs even if your first one doesn't, setting gravity back to 1500 thus overriding the first set gravity to 0. Moving one pixel at a time is not fast enough to fight against that gravity (moving 50 was).