There seem to be two types of argument going on. The first is an argument from causality and is almost a reductio ad absurdum. If you accept in principle that everything must have a cause, then you can follow the chain back ad infinitum and never find a starting point. This strikes me as similar to arguments over the existence of the universe and whether something can come from nothing. Although it's an interesting argument, it doesn't really help because I don't look upon free will as a problem of mechanics. I don't accept that all thoughts have to have a single "cause", the process is far too complex to be suitable to that kind of simplistic reasoning.
The second argument redefines free will in terms of human motivations, Schop. says that we choose to do what motivates us most, and that as we don't choose our motivations, we don't in effect have freedom of choice. I also find this argument very unsatisfactory.
My take on free will is that it is neither a question of cause or of motives but is an issue of absence of limitations on our choices. I accept that we all have motivations, but we are also able to have a rational discussion about these and reach a conclusion, and there seems to be nothing to stop us from doing this, other than facing the consequences of those choices.
No doubt most choices are predetermined in the sense that they stem from motivations, but we seem to have the ability to influence our motivations by lending more weight to some than others. We are not just influenced by motivations, we also take into account what the consequences will be and make a rational judgement about the implications of making one choice or another. For me, free will is a conceptual framework which captures this process using two words.
For there to be free will under Schop.'s definition we would need to be able to break the chain of cause and effect or change the laws that govern the universe and I'm not sure anyone really thinks this can happen or indeed is really what we mean by free will.
It seems that there must be a way to define 'free will' not in terms of the cause or motivation for an action, but instead in terms of the ability to assess the consequences of different choices and attribute moral value to those choices. When we couple this with the lack of constraint on this process, we end up with free will. If a rational person or persons make a normative choice which could have been otherwise (i.e. we can easily conceive of them making a different choice) then that choice has a moral value, it is in effect a case of "free will".