"tried reading Mattick's book Marx and Keynes once, and found it absolutely pointless"
Well, granted it is better as a restatement of Marx than a critique of Keynes, but given it was published in 1969 and the book's central thesis was that the post-war boom did not herald the end of the business cycle, but was on the contrary the state-led reconstruction of capitalism about to reach its limits, I would have said it was rather to the point.
"There's nothing necessarily militaristic about Keynes or Keynesianism - it's deep in the bourgeois state."
I don't think Mattick said that Keynes himself was inherently militaristic, but he did think that it was because the reconstruction was on a national basis that it was to be expected that it would lead to national conflict. In Economic Crisis and Crisis Theory he argues that the adoption of national recovery plans only displaced the destructive side of capital accumulation, so that the cycle of economic crises gave way to a cycle of world wars.
I never met the son (though I have read his ex's disturbing magazine article about stalking him), so I have to take your word - though he did write a rather good philosophy book called Social Knowledge (1986).
Mattick's group did follow a lonely path, dissenting from the New Deal, and maybe that made some of their judgements sound a little shrill. But I am more impressed by how much they were on the ball. Mattick wrote analyses of various state's economies on some Public Works programme, and he republished his more tendentious anti-New Deal arguments in an essay The Great Depression and the New Deal, in Economics and Politics in the Age of Inflation:
"the Keynesian theory found no verification in the New Deal. The depression was finally ended not by a new prosperity but through World War II, that is through the colossal destruction of captial on a worldwide scale and a restructuring of the world economy tht assured the profitable expansion of captial for another period. National solutions tothe economic crisis had everywherefailed, but by attempting such solutions, the capitalist world system, already shaken to its foundations, had still further deteriorated. Imperialistic solutions were now the order of the day, not least because of the quasi-autarchic anti-depression measures that preceded the war." (in Economics and Politics in the Age of Inflation, Merlin Press, p 141)