You need to show that the value in question is the best way of generating happiness.

You're absolutely correct. Discovery may not always be the best way of generating happiness; and if it's not, you should do something else.

And the other point is that values aren't supposed to do a job.

Not all values are terminal values. Some people value coffee because it wakes them up; they don't value coffee in itself. If they discover that coffee in fact doesn't wake them up, they should stop valuing coffee.

With the examples of discovery and creativity, I think it's obvious that they increase happiness by a lot.

The point is that's not sufficient.

What is sufficient is demonstrating that if discovery does not promote happiness then it is not valuable. As I explained in my sorting sand example, discovery that does not in any way promote happiness is not worthwhile.