...in order for a (fictional) reason to have normative force, we must treat it as authoritative. To treat fictions as authoritative is to commit oneself to the valuing game. A inclination to stop playing the game, if it is to have force, must be endorsed as normative. And to endorse it as normative is to play the game. So the fact that it is 'only' a game will not cause us to stop playing it, even when our inclinations are against it. They only count against when we are affirming it.
I originally intended to ask a question about whether this really does justice to the psychology of the person suffering depression from nihilism, but Simon Blackburn asked it before I did. So I asked a variant of the question, namely, whether this does justice to the psychology of the happy nihilist. Think of a Romantically-conceived Prometheus, viewing the Law of Zeus. Naturally, it's the first Law of Zeus that One must obey Zeus; but Prometheus sees through all the pompous pretensions of Zeus to authority and normativity, and finds it funny. The above argument doesn't really seem to deal with such a nihilism. Ichikawa suggested that the real issue is practical reason; unless we admit some sort of normativity or authoritativeness to reasons, we can't act.
I am much, much farther away from nihilism than Ichikawa is -- almost as far away as one can be -- but I think the nihilist is on much stronger ground than this. When I am thirsty, I don't need a normative or authoritative reason to drink a glass of water; I just need an inclination to do so (which I do, ex suppositione). Likewise with the rest of life. There is a sort of normativity or authoritativeness associated with reasons; but this is just the straightforward fact that a person can rationally propose a standard of behavior and hold himself to it -- and that, really, is all there is to normativity. We have something identifiable as a norm, and we treat it as a norm by expecting it to be upheld. But this is basically to say that the person himself has an authority over his acts inasmuch as he is the natural originator of them, and not much else; it does not follow that it is necessary to practical reason that his reasons have authority over him. The mistake, I think, is thinking that values are necessarily normative, which they are not. A nihilist is not being inconsistent in valuing things and yet considering it pompous, pretentious, or irrational to attribute authority or normativity to the value. It would be utterly absurd for me to attribute some 'normative force' to my reason for acting on my inclination to drink water; I just have a reason to drink, and so I drink. And I could, potentially have something I consider a better reason not to drink, not because there's something called 'normative force' that one reason has in greater quantity than another, but simply because I like it better. Ichikawa is addressing the sort of person who is inclined to say that he acts not out of 'normative reasons' but 'because he just feels like it'; but, I think, 'I just feel like it' is actually able to cover an immense range of human behavior.
Ichikawa gives some rational credit to the nihilist on the basis of Mackie's argument from queerness and Harman's argument from moral explanation; that is why he proposes a value fictionalism rather than a value realism. Since I think both arguments beg the question and are based, moreover, on necessarily false premises if my own theism is true, I don't give the nihilist such credit. If Mackie's and Harman's arguments were to succeed, however, I think the nihilist is immune to Ichikawa's argument; the nihilist is still quite capable of acting. There are lots of things we do that we do on the basis of no reason except that we felt like doing it, a reason neither normative or authoritative unless one makes it so; and the nihilist is just extending this to everything. Reasons are not 'normative entities'; they don't need to exert 'normative force' or 'prescriptive force' on us; they just need to be had and acted upon (if we feel like acting on them).
There is a place for normativity in practical reason; and I think a full account of practical reason requires it (and I think defenders of Mackie and Harman are unreasonable in not realizing that this is so, and in a way independent of the quite arbitrarily-chosen extrinsic standards to which they insist it conform to be admitted). Not, however, in order to act, since the nihilist has an account that can cover an indefinitely large range of human action. The problem with nihilism is that it is irrational, not that it is impractical.