Unfortunately zones aren't really climates. They are simply share minimum winter temperatures.
You can pretty much grow any kind of pine or spruce in the San Francisco or Santa Clara.
Yes you can start the air layer any time.
For the cutting humidity, I don't know, it would probably depend on the air temperature. How about as humid as you can get that doesn't condense.
Here is a link that might help
http://www.ces.ncsu.edu/depts/hort/hil/hil-8702.html
Regards,
Matt