I think you miss something in regard to heterodox economics. Keen and Varoufakis maybe are not there, but Wray, Mitchell, Kelton an others would suggest that MMT is a way of looking at the political economy an overcoming these hurdles. The way it is use can be from a socially progressive view or not, but it does explain the structural problems that the neo-liberals trap us in, for their own benefit. Their economics is their ideology, not an explanation. Marxists can use benefit from an understanding of MMT to develop a radical alternative too. Your book an writings are a great source of ideas on all this.