Does clamshell mode reduce lifespan due to heat? I been playing some games on my rMBP in clamshell mode as having 2 screens causes framerate to drop and I don't want the CPU/GPU to render a screen on laptop side that I don't use. 

My main concern to this approach is the heat generated, does clamshell mode trap more heat? will the heat damage the laptop screen if used for long periods of the time?
I doubt that the heat will damage the screen, but considering how MB and MBP feed the fan, I'd be concerned more of airflow cutoff as lots of the air comes through the keyboard itself. You could look into placing a small magnet over the reed switch area and leave the lid open, it's near the Battery Indicator Light. Be careful though, don't swipe a magnet over your HDD.