All by itself, the clock does not influence the longevity in any way.
However, power consumption and dissipation rise linearly along with clock speed and those two factors are the chip killers when the microscopically small traces inside the chip "burn up" (actually, it's electromigration that kills them - a process where electric current displaces molecules of the conductor it's running through).
Load (heat and current) also leads to degradation of transistors inside the chip to the point where they become unstable at speeds they used to run at without any issues. If your card suddenly starts crashing where it used to run stably for months dropping the clocks is the only solution.
95% fan speed (or even 85%) is something you should worry about. When manufacturers cut corners, fan quality goes first.
Chances are that when purchasing inexpensive 10$ case fans you are getting much higher quality devices than the ones mounted on your 300$ card...
70..79°C is not a bad temperature range for a battery of overclocked 6970s locked in a PC case. If it's an open rig, try increasing the distance between cards or pointing more airflow directly at them.
It's a bloody damned shame that your manufturer seems to have cheaped out on the VRMs - undervolting should drop the temperatures just enough for you to be able to reduce the fan speed to sane levels.
(1) && (2) No clock - all by itself - is too high provided the card is stable. 79°C isn't exactly low but it'll do.
(4) Independent of current and temperature, no.
Thanks for the input.
The voltage is strange on the 6970s -- I can't adjust voltage on any of the cards, but one is at 1.150 and the other two are at 1.175. Go figure.
They are all locked in the case but I'm likely going to open the case and direct a desktop fan towards it.