Best combo for macro using extension tube?
I have never found AF and macro to mix well on the 5DII, and almost always shoot with manual focus (nodding the camera bad or forward to achieve focus). The 7D (and presumably 5DIII?) has special support for AI-Focus tracking and seems to work well with macro.
If you need more working distance, you will need to use longer focal lengths. The problem is that to increase magnification you need proportionally more extension tube length as the focal length increases, and this eventually becomes impractical.
I have had good success with a Sigma 50mm lens plus 25mm tube (really surreal out of focus areas) and also with a range of lenses including the 100mm macro and 100-300mm zoom using a Raynox DCR250 close-up filter. I have never had much success using extension tubes and shorter zooms - partly because either the working distance becomes too short or because the number of tubes becomes unfeasible impractical.
So I would suggest you look at a close filter for your 70-200mm. It will give you both reasonable magnification and working distance, and it can also be used with your multiplier. You might also want to experiment with a flash with an off-camera trigger (eg cord) , as it can be quite hard to freeze motion/shake at higher magnifications.