1, Corona does not support non-linear workflow, so you need to work in linear color space...
2, Motion blur will always have some overhead. Only workaround i can think of at the moment is rendering motionblurred curtain on the black BG and compositing it in, or doing the blur in post, or just waiting longer.
3, That's what translucency is for.
4, You do not anyhow tweak contact shadows. Corona is a path tracer, that means it simulates how light behaves in real world. If you take a look at the photos, they do not look like like full of ambient occlusion. Light reflects, AO is just a very simple trick that does not take light bouncing into account. That being said, you can render out AO pass if you want, using Texmap render element.
5, There are better ways, like hair&fur or grass-o-matic plugin.
6, I suggest first using LWF to see how your scene looks with correct texture gamma, and then re-evaluate scene and go further.