1) I don't see a definition of disabling a zone. Am I correct in assuming this means that they can't be activated?
You are correct.
2)Can a power plant in a disabled zone be activated (as it can in an inactive zone)? (I'm guessing yes)
No he cannot - it is like this zone is unconnected to the rest of the city. You will need to "spend" a movement decision to move or to just wait to the end of this Disaster effect.
3) Turned over buildings are not only not available for activation, but also not available for favor scoring, correct?
That is true.
4) Most disaster cards referencing buildings say to turn them over. But the Crash Landing card says to disable all safety buildings. I see two ways to interpret this:
a) It's a typo, and should read "turn over all safety buildings"
b) Disabled safety buildings can't be used, but are still available for building favor scoring.
I'm guessing a) is correct.
b is the correct answer - you cannot active them but you can score favor - no typo here