Well, the Q6600 OC's like a dream--most of the time. I've built 4 PC's using the Q6600 simply because it is darn near effortless to OC, and it's been a strong, reliable chip.
1. Unless you do something rash like overclock at higher-than rated volts in the middle of summer in Death Valley, the lifespan of the chip won't suffer any undue lifespan shortage. That said, I've got a 25% OC on my current Q6600 while undervolting it.
2. Depends entirely on cooling, the chip (also the stepping), the ambient temps, etc. Generally speaking, a good aftermarket cooling solution gives you cooler temps which translates into more room to OC.
3. At stock speeds? A good (ED: overclocked) Q6600 will annihilate a stock Q9550. All things being equal, I believe (correct me if I'm wrong here, btw) that a Q9550 only gives you around 7-15% more in real world performance boost over the Q6600.