b) behind a fence
b) is only fixable by getting right up close to the fence and opening up the aperture as wide as you can.
I'm a real newbie when it comes to motorsport photography, only shooting the stuff about once a year, but FWIW this was shot at Silverstone with my 40D and 100-400 at 400mm, f/10, 200 ISO, 1/320. It's been cropped a bit and levels adjusted but that's the only editing. I'm sure there was a fence in the way.
One more, again at Silverstone, through fencing, at 400mm, f/5.6, 1/1000, 200 ISO....
The problem with opening up the aperture is that it's hard to slow the shutter enough, in good light, to achieve decent motion blur. On a sunny day a CPL or ND filter might help enhance the sense of motion when panning, while still allowing a wide aperture.
Here's one more, this time shot through a gap in the fencing at 200mm, f/7.1, 1/400, 200 ISO. Note how little panning blur there is at 1/400...
One more thing about the 100-400 - Mode 2 IS should help with the panning success.