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It is the monsoon season here in Northern California, and my “shop” is my garage. Which of these activities are safe, or should be avoided during high humidity days?

1. Thicknessing top, back, sides either by sanding or with a plane
   Bending the sides
   Joining the top/back pieces
   Radius sanding the top and/or back
   Gluing Kerfing
   Sanding a radius into the braces
   Gluing the braces
   Carving the braces
   Gluing the neck and tail blocks
   Shaping the neck
   Fretting
   Gluing truss rod in place
   Gluing the fingerboard in place
   Positioning and gluing the bridge
   Basic Setup

Randall, we aren't getting near as much precipitation here in the central valley as you are up there, but my shop is in my garage too. I have built 12 guitars there, and never worried too much about the temp or humidity. I've done all those things on your list at all times of the year. Since I can't control the humidity in my shop, I don't worry much about it. Gluing up anything when it is 110 degrees though is tough to do as the glue skins over before you can put the pieces together!!
I think you are pretty safe with any of those things. If your top plate stays flat while you are gluing braces, you should be good to go.
That is just my own opinion, but from my limited experience.

Like Kevin, I have never worried about it and had no adverse effects, but with monsoon going on, I might delay gluing braces to plates until things dry out a bit, The problem is that the wet plates will dry out and flatten any dome you have, or can potentially crack a plate if there is no dome

do you have a relative humidity gauge? I live in Seattle and it rains and rains, but the relative humidity inside is usually not high. If you heat the cooler outside air at all the relative humidity will drop because warm air can hold more water than cold air. I am not saying the relative humidity in you garage is low or high, but it would be worthwhile checking it out with a meter.