MacBook Air CPU core PECI temperature
Here are some things to keep in mind for your streaming event. 


If the CPU gets too hot, the MacBook will throttle and reduce clock speed. That could result in dropped frames. Most of the time you will hardly notice the difference. 
iStat Menu has a utility to override fan speeds. In your case I would consider cranking up the fan to full speed during the streaming event, unless the noise is an issue. 
During the event put something underneath your MacBook so air can circulate below it. That doesn't do miracles but results in 1-2 degrees less and therefore less risk of throttling / dropped frames. 


Macs have pretty sophisticated cooling design and CPU temperature is not the only aspect taken into account for controlling fan speed. I would suggest you do another dry run for 2-3 hours. If you don't have issues during that time 12 hours should work as well.