I am happy with the way you characterize my beliefs in the first paragraph of your blog. Unemployment is clearly not Pareto optimal. Everything you say after that is at best misleading and at worst dismissive of everything we (at least some of us) learned from Keynes.
The idea of involuntary unemployment was introduced by Keynes in the General Theory. But you already knew that. It is defined as a situation where (in modern language) the ratio of the marginal disutility of work to the marginal utility of consumption is not equal to the real wage. That seems a pretty accurate description of the equilibrium outcome of labor search models.