1. Yes; 10C reduction in temperature means doubling of life. I've known pentiums to last 10 years at 60C+; just running processors at 30*C instead is 80 years minimum. Main thing is to use leaded solder so you don't get electromigration problems.
2. Solar panels and batteries. Battery voltage is chemical and fixed by physics; nickel-iron batteries can be rebuilt and last forever. Solar panels can be oversized to provide enough energy even when they degrade over time and/or the computer can just be used at a lower duty cycle.
3. Make it big and hard to move in a sturdy box.
4. Flash can last that long if it is periodically rewritten, kept cool, has redundancy, and isn't updated often.