Nice to see a well set out question. Easy to read and understand.
If you do anything to block the flow of air through the Cooling System your laptop will almost certainly over heat.
Many Laptops have Heat Exchangers built into their base plate. If you restrict the airflow over this, the poor devil will over heat.
So: Best place your laptop on a flat surface.
I suspect that if the laptop is at an angle, say 30 degrees off the horizontal, natural convection will increase the airflow over the heat exchanger and therefor lead to a cooler machine, but I don't know anyone else who shares this opinion.