Well proper computers have various intake and exhaust slots in the main body, so a laptop cooler will actually be blowing air into the system. However, Apple like to wrap their toys in enclosed aluminium shells so you may have problems with standard laptop coolers.
It can only damage it if its running dangerously hot, otherwise running a processor with 100% usage will not kill it.
If a processor can not run at 100% for 3 years (retail) or 1 year (OEM) while keeping at safe temperatures then it is faulty. You don't buy hardware expecting it to only handle light loads Heat will kill it though, so keep that in check.
if your really paranoid you could always down clock it, reducing heat though obviously it will take a lot longer to render. But I wouldn't bother. Just make sure your pc is resting on a hard surface rather then a soft one and i'm pretty sure that resting it on a heat conductive surface would help as well
Rendering will be OK; however blendering is not recommended .
your cpu is working while rendering?so its doing what its designed to do