Basically, it wouldn't be a puzzle, if I put a 2 metres long pink key into the yellow sand

As I said, when you are at that point, you know that you must search a key. There is only 5 screen, with very few objects, and you have a "magnetic cursor" what helps you find the key. As I said, some people noticed the key in 2 seconds, and others found it in some minutes. I'm sorry if you had problems, but I'm not planning to make it easier :/