Patrick writes about capitalism 'exploiting non-capitalist spheres' - I thought that Jairus Banaji dispensed with this argument some years ago, when he explained that just because capitalism had not reorganised production on capitalist lines, it did not meant that the peasant production (in the Deccan, in his example) fell outside of the capitalist mode of production. I think he was right that capitalism was a world-wide phenomenon by the 1850s, and that uneven development was a facet of capitalism, itself.
Also, Mattick makes a good logical argument that Luxemburg's dependence on 'non-capitalist' spheres as a way of explaining the limits of capitalism efffectively means that she discovers the limits of capitalism, outside of capitalism. (Whereas Marx argues that capital itself is the barrier to capital accumulation.)
Carrol has a point about imperialism. You cannot go on insisting that we are at the highest stage of development of capitalism forever. Also, the crux of Lenin's theory of imperialism was that it was an epoch of transition. Without the subjective factor of a movement towards transistion, there cannot be an epoch of transition. The defeat of the working class opposition in the 1975-1985 period was the condition for the survival of capitalism (most obvious in the transformation of the former Stalinist bloc).