So, the idea of free will is not the same as free will, and free will is not an idea. It is what the idea of free will is the idea of.
That is pure nonsense. Needless complications risen from the inacurate use of words.
But it goes directly towards what I mean.
Since you chose to speak of the validity of the concept free will in term of existence we have to have this huge debate about ideas and the things themselves, invoking a whole area of philosophy that may be unneccesary for providing us with clarity in the issue. Just because you want to justify a category mistake. We have to create the distinction between an idea and the contents of it, that make the idea, which is a complication that only serves to confuse, in my opinion.
If you had asked wether free will is a valid concept to human perception, it would have saved us all that trouble. If you had taken the time to really understand what the word means. One word can be food for philosophy for a very long time.
Now we have the explanation that free will doesn't exist, but the idea free will exists. This subtle difference in meanings doesn't allow us to easily find clarity, does it? There are simpler ways to say this, using more appropriate words, wouldn't you say?
And as to unicorns. I have never come across an explanation of unicorn that stated they were creatures that exist in the physical world. So both the concept unicorn, and the creature itself, as it is defined and understood, also exists, in the pages of a fairytale somwhere, roaming the green hills of some imaginary country.
When you say they don't exist, you are assigning an attribute to them that wasn't originally intended for them, usually such a satement is a reply though, to someone else who has already done this assigning.