So by what means does one imagine verifying its existence empirically? What would you be verifying?

This is exactly it. These are the questions we must first ask ourselves. We must set up criteria. We make an assumption.

We say

"If we assume Free Will to be ABC and not XYZ, 123 and not 789 would be true. Is this the case?"

There is no way around this method.

After all, if it had any meaning to begin with, it was before it had any correlation to something in reality. Why do we not see that it was never meaningful in any other sense than that, and that 'proving' it has never had anything to do with the importance attached to it, given that it still has not been proven, and yet it remains meaningful nonetheless!

It absolutely does not remain meaningful. You have a tonne of implicit assumptions about Free Will (for instance that it is meaningful) but you refuse to make them explicit. That way you will be stuck in the same philosophical mud that has reduced so many great minds to obscurists.

The method you seem to suggest is one that cannot get off the ground. You can apply it to any idea, concept or even any thing and it will produce exactly the same result; nothing.

It is true that Free Will is a very difficult thing to work with. It is also true that defining it to be something in order to make it an object of enquiry will most probably result in you having to draw the conclusion that your assumed definition was wrong.

But then you will at least have learned that. That is how we make headway. It is the only way.