MacBook – Where does the cool air come from

You are correct. While the major fan vent is in the back, it is important that air can circulate under the laptop. This is because the bottom side of the laptop can get hot, and the space between the laptop and the (hard) surface helps release that heat. A soft, squishy surface will just trap it and push it right back up into the computer.
And a computer with only one fan vent needs all the help it can get.

That said, you should be OK using the laptop on something soft for most of the time. It's when the fan starts spinning that you really should move to something more solid.
To keep an eye on how hot your computer is,\ and how fast the fans are going, I use the free iStat Pro widget.

Hope this helps.

If you don't need to use the MBP screen while using the external monitor you can disable it using a small magnet.

A small magnet placed on the left side of the MBP keyboard, more or less above where the SD card reader is placed, will make it think that the lid is closed. The GPU will need to do less work to drive just one screen (the external) and use less power, and with the lid still open the airflow will be beter.

I use this on a 2011 MBP, other versions may need to have the magnet placed at different places for it to detect a closed lid. And if you try this, please use the smallest magnet available.