If JBJ is offered only $5mill in arbitration, the two sides will not agree and the Sox will lose the hearing.
In arbitration cases, each side submits a figure, and an independent arbiter decides which of the two salaries the player deserves. He chooes one of the two offers, not some third number comprimise. If the Sox offer $5mill, Scott Boras will counter with $12mill. Then the arbiter will definitely decide the $12mill salary is the one Jackie deserves, because Bradley is going to get paid closer to AJ Pollock than he is to Jon Jay.
If the Sox don't offer arbitration, then Jackie is a free agent. That's how it works. And Boras knows JBJ can get a better deal in free agency than that, and he will advise Bradley accordingly. And Bradley didn't hire Boras so he could ignore his advice.
That is how those "ifs" will play out...