1. I think there's a bit too much force when the grappling hook pulls you up. I was able to use it to fling myself upward, where I could double jump and just go flying. A cooldown might make it more difficult, but it could slow down gameplay depending on when the cooldown starts. I could see a cooldown from the moment you send out the grappling hook being good, as it will stop you from just spamming your way to the top.
2. Maybe some falling rocks/candy might be good, and parts of the wall that you can't grapple so you have to be more careful how you climb.
3. It's too slow. I was able to go so far ahead that when I decided to fall, it took a solid 3-5 seconds to find the milk.