How would you convince others of the "iceburg" and how they can obtain such knowledge? I think that was the whole point of determining free will? According to Warpus, you have to give some proof of your claims. It seems they deny free will although they have not provided any evidence other than there is none. The first step would be, even to agree what frame work you are even working in so they can agree with you on the definitions.
My ambition is just to present certain POV not to make any strong claims or to convince others but I surely must have some degree of conviction myself to hold these views so here is simple why:
I can observe three basic parts(physical body, emotions, mental faculty)of my own being which non of these I have really deep knowledge or supreme control on personal level. Yet logicaly if I want to use my personal will most effectively this is exatly what I need - maximal cooperation and harmony inside my own being (integrity). But I can also see that through discipline I can exert or expand level of my control, understanding and increase of my capacity. I can expand the "top of an iceberg" in self-conquest. So one of my main convictions is my practical experience.