The only unwrap that really matters is on the low poly. If you'll only use the high poly for surface/mesh detail Normal and AO then you don't need to worry about UV unwrapping the high. Only if you plan to bake the diffuse from the high, will you need UV's and even then they can be anything. It doesn't need to be optimal…
Mental Ray AO has very few settings and they're only found in the RTT window. MRAO doesn't factor in things like materials or lighting, its straight up proximity and angles between geometry. So the skylight and the materials you added was just pointless and you can stop trying to tweak that stuff. You have three options:…